Summary. Sexually mature rams were left intact, castrated (wethers), castrated and implanted with testosterone, or castrated, implanted with testosterone and pulse-infused every hour with LHRH. Serum concentrations of LH increased rapidly during the first week after castration and at 14 days had reached values of 13·1 ± 2·2 ng/ml (mean ± s.e.m.) and were characterized by a rhythmic, pulsatile pattern of secretion (1·6 ± 0·1 pulses/h). Testosterone prevented the post-castration rise in serum LH in wethers (1·0 ± 0·5 ng/ml; 0 pulses/h), but a castrate-type secretory pattern of LH was obtained when LHRH and testosterone were administered concurrently (10·7 ± 0·8 ng/ml; 1·0 pulse/h). We conclude that the hypothalamus (rather than the pituitary) is a principal site for the negative feedback of androgen in rams and that an increased frequency of LHRH discharge into the hypothalamo—hypophysial portal system contributes significantly to the post-castration rise in serum LH.
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