Summary. Testosterone, oestradiol and progesterone were measured in peripheral plasma during the oestrous cycle of 6 heifers. Oestradiol and progesterone results confirmed earlier reports. Concentration of testosterone on the day of oestrus was 40 \ m=+-\ 3 pg/ml (mean \ m=+-\ S.E.M.), and two peaks were detected during the cycle, one 7 days before oestrus (1809 \ m=+-\ 603 pg/ml) and the other (78 \ m=+-\ 7 pg/ml) on the day before the onset of oestrus. The concentration of progesterone declined in most cases 1 day after the maximum concentration of testosterone.Betamethasone treatment in 5 heifers extended luteal function by an average of 10 days: plasma androstenedione and oestradiol concentrations were unaltered; cortisol values were depressed for at least 16 days after treatment; testosterone concentrations were lowered by 13 \ m=+-\ 2\m=.\4%during treatment, and except in one heifer the peak on Day -7 was abolished.

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Plasma and pineal melatonin and plasma corticosterone and progesterone concentrations have been shown to be altered by several types of stressors. This study was designed to define the circadian patterns of the hormones mentioned above in rats subjected to chronic stress and to investigate the influence of constant illumination. The results revealed that melatonin and corticosterone circadian patterns deteriorated and their plasma concentrations were significantly elevated. The constant illumination (2,500 lux) during the dark period (from 2000 to 0600) was not able to suppress melatonin production in stressed animals, while the plasma content of corticosterone was decreased at the end of experimental period compared to control rats. Plasma levels of progesterone were increased in stressed animals as well. Constant illumination, however, provoked also an increase of progesterone secretion in controls. Statistical comparisons between hormonal secretory patterns showed that melatonin and corticosterone correlated negatively in controls (r = -0.58, P less than 0.05) during the nighttime. However, in stressed animals correlation was observed only between melatonin and progesterone secretion during the light and dark period (r = -0.43, P less than 0.05). Surprisingly, the correlation during the nighttime in rats subjected to constant illumination was negative (r = -0.60, P less than 0.02) compared to positive correlation (r = 0.60, P less than 0.02) in rats kept under normal lighting regimen. These results suggest that melatonin release is affected by stress and, possibly, under these circumstances, interacts with adrenal steroid secretion.