Summary. The

The levels of FSH, LH and testosterone in the plasma were measured in cross-bred male lambs which were bilaterally castrated, hemicastrated or left entire at 1 week of age. Removal of one testis caused the level of FSH in the circulation to increase to a value midway between the levels found in entire and castrated lambs up to 12 weeks of age. The concentrations of LH and testosterone in the plasma were similar in the control and hemicastrated lambs. The concentration of FSH in the plasma of the hemicastrated lambs fell at 12--14 weeks of age, the expected time of puberty, to the level found in control animals of the same age. Although the possibility of increased release of LH cannot be excluded, the increase in the secretion of FSH is believed to account for the compensatory hypertrophy of the remaining testis observed in the hemicastrated lambs. Removal of one testis from mature Soay rams in sping (May) caused a small selective increase in the concentration of RSH in the plasma, compared with entire animals. This difference was maintained until July when there was a marked increase in the levels of FSH and LH in the hemicastrated rams such that an exaggeration of the normal seasonal changes in these hormones was observed. Compensatory hypertrophy of the remaining testis was observed in the hemicastrated animals.

Hemicastration of rams at 1 week of age resulted in compensatory growth of the remaining testis. This was associated with an increase in the concentration of FSH in peripheral plasma which was maintained until the rams were approximately 10 weeks old. Plasma concentrations of LH were similar in both entire and hemicastrated lambs during this period. Pulsatile release of LH was observed in all rams from approximately 6 weeks of age onwards. The frequency of these pulses increased and the size of each pulse declined as the animals grew older. At 16 weeks pulsatile release was almost undetectable and the basal levels of LH had increased. The increased concentration of FSH in plasma was related to the size of the remaining testis but not to its development. The return of the plasma concentration of FSH to values similar to those found in entire rams preceded the appearance of a lumen or of any primary spermatocytes in the seminiferous tubules. Restricting the development of the remaining testis in hemicastrated rams, by trapping it in the inguinal region, did not alter the FSH response to hemicastration. The administration of gonadotrophin releasing hormone (10 microgram) at 10, 13 and 18 weeks of age provoked release of similar quantities of LH in both entire and hemicastrated rams. There was also a substantial release of FSH at 10 weeks of age but this response declined with age and by 18 weeks there was little FSH released in any of the animals. This loss of response occurred earlier in the hemicastrated rams. The data suggest that in prepubertal rams the release of FSH but not of LH is restricted by a substance produced by the developing Sertoli cells.

The concentrations of prolactin in plasma were measured before and after the injection of TRH in pregnant ewes and in non-pregnant ewes during the breeding season and seasonal anoestrus. During the first 80 days of pregnancy basal concentrations of prolactin were low and stable but thereafter increased progressively to reach maximum values at Day 140. During a comparable time of year (December\x=req-\ April) basal concentrations of prolactin in non-pregnant ewes were not significantly different from those found between Days 20 and 120 of pregnancy. At other times of the year basal concentrations of prolactin in non-pregnant ewes were elevated during anoestrus and declined markedly at the onset of the breeding season. In pregnant and non-pregnant ewes the responsiveness of the pituitary gland to stimulation with TRH was significantly correlated with the basal concentration of prolactin in plasma: the raised concentration of prolactin at Day 140 of pregnancy and in July in non-pregnant ewes was associated with the greatest release of prolactin from the pituitary gland. A change in the responsiveness of the pituitary gland may play an important role in the overall control of prolactin secretion in the ewe.

Department of Physiology & Biochemistry, The University, Whiteknights, Reading, RG6 2AJ (Received 29 November 1977) Testicular fluid (RTF) collected from rams in this laboratory has been shown to contain 'inhibin', which selectively inhibits the release of follicle-stimulating hormone (FSH) in rats (Franchimont, Chari, Hazee-Hagelstein, Debruche & Duraiswami, 1977). During these collections, the possibility that diversion of RTF from the remaining testis of hemicastrated rams might provide further evidence about the feedback control of FSH was examined. Eight mature Clun Forest rams, hemicastrated at least 3 months previously, were placed in metabolism crates in a room with natural lighting for 7 days, after which the extratesticular rete of the remaining testis was cannulated (Voglmayr, Scott, Setchell & Waites, 1967) under general anaesthesia. Each ram was returned to its crate, where it was supported by a canvas harness during the collection of RTF (Edwards, Dacheux & Waites, 1976). The operations were performed