Serum levels of androgen were higher during the late part of the light phase of the light: darkness (LD) cycle than during the late part of the dark phase during three consecutive days in intact male rats. The serum concentration of androgen in blood samples obtained at hourly intervals from individual male rats fluctuated markedly with occasionally as much as a tenfold difference between the highest and lowest values. Evidence was presented that endogenous pulses in the concentrations of androgen in serum of male rats occur during the late part of the light phase and the early and middle parts of the dark phase of the LD cycle. Induction of sexual behaviour by injections of testosterone in castrated rats depended on the time of injection, and the period of maximum behavioural sensitivity to testosterone coincided with the time of the LD cycle when pulses in androgen levels in serum normally occurred in intact rats. Injection of testosterone produced extremely high concentrations of androgen in serum within 0·5 h of injection but these levels declined rapidly thereafter. Testosterone implants, which produced low levels of androgen that did not vary with the LD cycle, were considerably more potent than testosterone injections in stimulating sexual behaviour in castrated rats.
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