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Sequential blood samples were obtained at hourly intervals from intact male rats during various phases of the light: darkness cycle. Measurement of the serum concentrations of LH and androgen showed that both hormones were secreted episodically, with a secretory episode defined as an increment and subsequent decrement in the serum hormone level exceeding the analytical intra-assay imprecision. Episodes of androgen secretion varied greatly in amplitude (from 1·04 to 30·90 nmol/l) in individual males, occurred during any phase of the light: darkness cycle and were preceded by one or two episodes of LH secretion. Individual males could show more than one episode of androgen secretion during an 18-h sampling period (mean ± S.E.M. = 1·7 ± 0·7 pulses/10 h). Mean values of serum LH and androgen in groups of animals obscured the episodic pattern of hormone secretion shown by individual animals.

Castration reduced serum androgen to non-detectable levels within 2 h and produced a temporary decline in serum LH levels and abolition of the episodic pattern of LH secretion. These effects were prevented by testosterone treatment at the time of castration. There was a gradual increase in serum LH levels and in the amplitude of the LH secretory episodes for 1 week after castration.

Treatment of castrated rats with testosterone-filled constant-release implants (10 or 20 mm long) produced high and stable serum androgen levels and episodes of LH secretion of low amplitude 1 week after castration. A constant concentration of serum androgen comparable to the mean level of intact rats, produced by implantation of 5 mm long testosterone implants 3 weeks earlier, resulted in a pattern of episodic LH secretion which was similar to that of intact rats.

Subcutaneous injection of various doses of testosterone (5, 15 or 45 µg) in castrated rats produced a dose-dependent increase in serum androgen levels within 30 min of injection and thereafter the levels declined. Injection of 15 µg testosterone produced a pulse-like increase in serum androgen concentrations with an amplitude within the range of that observed in intact rats. Injection of this amount of testosterone in castrated rats in which serum LH levels had been suppressed by prior implantation of 20 mm long testosterone implants produced no change in serum LH levels. It is suggested that androgen primarily modifies the amplitude of the LH secretory episodes and that episodic fluctuations in serum androgen levels have no immediate effect on the pattern of LH secretion. Constant serum androgen levels comparable to the mean level of intact rats are sufficient for maintenance of the normal episodic pattern of LH secretion.

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