1. A radioimmunoassay for ovine prolactin is described based on the inhibition of the reaction between (131)I-labelled ovine prolactin and guinea-pig or rabbit antiserum to ovine prolactin. The extent of the reaction after a 4-day incubation period is determined by chromatoelectrophoresis or by adsorption of unchanged (131)I-labelled ovine prolactin on charcoal. The sensitivity is equal to 5.9ng. of prolactin/ml. of plasma with chromatoelectrophoresis, or 0.2ng. of prolactin/ml. of tissue extracts with the charcoal separation. 2. A complete cross-reaction demonstrated between ovine prolactin and caprine pituitary extracts allows the assay to be used to measure caprine prolactin. The partial cross-reactions between ovine prolactin and bovine prolactin and between ovine prolactin and bovine pituitary extract differ, and an alteration in the immunological activity of bovine prolactin during its isolation is suggested. Bovine prolactin in plasma may be measured against a bovine pituitary extract as standard. No cross-reactions were demonstrated with pituitary extracts from a number of other species. The extent of the contamination of ovine and bovine growth hormone preparations by their respective prolactins is shown. 3. Dilutions of ovine and caprine plasma inhibit the reaction between (131)I-labelled ovine prolactin and antiserum with the same characteristics as ovine prolactin. 4. The immunoreactive material in plasma fractionates on Sephadex G-200 and in sucrose density gradients as a single peak similar to that shown by freshly dissolved ovine prolactin. There is no evidence that ovine prolactin is bound to a plasma protein. 5. By suppressing prolactin secretion and assaying serial samples of plasma thereafter it is shown that the immunological activity of the surviving hormone becomes progressively altered with time. It is suggested that this alteration is usually not detected but introduces an element of uncertainty into the quantitative but not the qualitative value of the measurements obtained by reference to standard ovine prolactin.

Results are reported of measuring prolactin in goatplasma by a radioimmunoassay.Changes in concentration in plasma prolactin over time scales from minutes to hours were noted in animals not subject to experimental stimuli. It is presumed that these changes reflect the rapid removal of prolactin from plasma (t ½ 19 min) and the secretion of prolactin in response to unknown internal stimuli. Stimuli are reported which cause rises in plasma concentration suggestive of bursts of secretion from the pituitary. Copulation, in both males and females, sexual excitement in males and milking caused the largest increases in plasma concentration. Oxytocin injections and mild stress caused significant but smaller increases in plasma prolactin sometimes seen as a rise in jugular vein concentrations only. Plasma prolactin fell on fasting and after insulin and it rose on refeeding, but there was not a direct correlation with changes in blood glucose concentration.

Studies ofthe effect of mating on the release of prolactin over a 47-h period from the onset of oestrus in the ewe are presented. Surges of prolactin occurred around the time of pre-ovulatory luteinizing hormone release, and in some instances copulation appeared to result in an additional surge of prolactin secretion.

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