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SUMMARY1. Plasma concentrations of cortisol and corticosterone measured by competitive protein binding in rats between 5 and 28 days after birth have been related to the intestinal uptake of [1251]polyvinyl pyrrolidone (PVP).2. Plasma cortisol concentration was consistently low throughout the period studied, but there was an increase in plasma corticosterone concentration at the time (18-21 days) when PVP uptake declined to zero (closure).3. Injection of a large dose of cortisone acetate 5 days after birth resulted in precocious closure; PVP uptake declined progressively to zero during the 6 days following the injection. Injection of this steroid at 12 days of age caused closure within 4 days.4. Precocious closure induced by cortisone acetate was closely comparable histologically with natural closure; the decline in PVP uptake was associated with the progressive displacement of vacuolated cells from the villi of the terminal intestine.5. Injection of corticosterone at either 5 or 12 days after birth also reduced PVP uptake. However, the reduction was transient and uptake returned to control levels some days after the injection.6. The temporary reduction in PVP uptake following corticosterone injection was not associated with any change in the histological appearance of the small intestine at the light microscope level.7. The injection of either cortisone acetate or corticosterone was followed by a period of impaired body growth and also a reduction of adrenal weight in animals injected at 12 days but not in animals injected at 5 days.

SUMMARY1. A number of cardiovascular and endocrine responses which occur during and after feeding in the unweaned calf are described.2. There was a substantial increase in both heart rate and mean aortic blood pressure during feeding in these animals. This occurred within the first few seconds and persisted throughout the period of ingestion.3. The concentrations of glucose, insulin and gastrin in arterial plasma rose abruptly during, or immediately after, feeding and elevated values persisted for at least 2 hr. A transient increase in glucagon concentration was also observed. In contrast, feeding appeared to produce no immediate rise in enteroglucagon concentration.4. The adrenal output of glucocorticoids rose transiently in response to feeding but that of catecholamines was unaffected. 5. Cardiovascular responses to feeding were also examined in other species. In unweaned kids the changes were essentially similar to those observed in the calf but were less pronounced. In lambs a persistent hypertension occurred which was associated with a brief initial tachycardia. In adult dogs ingestion of solid food also caused tachycardia but although the aortic blood pressure rose for a short period at the beginning of feeding, hypotension developed thereafter.6. The possibility that both the cardiovascular and endocrine responses, which occur during or immediately after feeding, are mediated by the autonomic nervous system is discussed.

SUI[MARY 1. A technique is described by which the whole of the effluent blood from the right adrenal gland can be collected as required from conscious, unrestrained calves. The technique may be used to measure adrenal blood flow gravimetrically and to compute the output of adrenal hormones under various conditions in the normal calf.2. In a group of seven calves mean cortisol output from the right adrenal gland was found to vary between 20 and 40 ng. kg-' min-' and corticosterone between 6 and 18 ng. kg-' min-' during a 2 hr period, 24 hr after surgery.3. Intravenous infusions of synthetic adrenocorticotrophin (5 ng. kg-' min-') produced a significant increase in the output of both cortisol and corticosterone within 5 min. The output of both hormones rose to maximal values within 10-20 min and mean values of approximately 300 ng . kg-' min' (cortisol) and 120 ng. kg-' min-' (corticosterone) were maintained thereafter for the duration of the infusion (120 min). The output of both steroids fell to values comparable with those observed initially within 45-60 min after the infusion was discontinued.4. These changes in glucocorticoid output in response to adrenocorticotrophin produced a significant rise in the concentration of both cortisol and corticosterone in peripheral plasma. It is noteworthy that the rise in the mean corticosterone concentration in the peripheral plasma was substantially less than that which might be expected from relating the rise in mean plasma cortisol concentration to cortisol output.5. The results of control experiments have eliminated the possibility that the sampling procedure might itself increase steroid output or peripheral plasma concentration. Comparison of results from calves of widely disparate ages (8-38 days) provided no evidence that either the resting A. V. EDWARDS AND OTHERS output of cortisol or corticosterone or the response to adrenocorticotrophin changes with age within the range examined.6. Infusion of adrenocorticotrophin (5 ng . kg-' min') also stimulated an abrupt rise in adrenal blood flow; mean resting flow (210 + 23 ,dl. kg-') increased by approximately 30 % within 5 min and attained peak values (355-365 /zl.kg-1 min-) between 10 and 30 min. Thereafter, adrenal blood flow steadily decreased and then fell rapidly to within the resting range when the infusion was terminated. No significant changes in heart rate or aortic blood pressure occurred during these infusions.7. The results are discussed in relation to those obtained in other species and under differing conditions by other workers.