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Summary. Mean serum testosterone concentration in the 27-day-old prepubertal Holtzman rat was 0\m=.\25ng/ml, much lower than that obser- ved in the adult male, 5\m=.\5ng/ml. Within 15 min after the intravenous injection of rat LH (in a dose of 400 ng-equiv. NIH-LH-S1/100 g body weight) serum testosterone concentration in the prepubertal rat increased approximately fivefold. Maximal elevation of serum testosterone to levels as high as those observed in the adult male occurred 1 hr after intravenous injection of rat LH. Serum testosterone returned tocontrol' levels within 2 hr of the intravenous injection of a small dose of rat LH. A dose-response relationship existed between serum testosterone and the dose ofrat LH injected intravenously, over the range 25 to 1600 ng-equiv. NIH-LH-SI/100 g body weight. Until recently, assessment of the responsiveness of the prepubertal rat testis to the effect of gonadotrophins with respect to stimulation of steroid secretion usually required measurement of weight gain of the prostate gland or seminal vesicles at a long period of time after chronic administration of the gonado¬ trophins. With the advent ofsensitive radioimmunoassay methods for measuring serum levels of testosterone, such as that developed in our laboratory (Coyotupa, Parlow & Abraham, 1972), it has become feasible to evaluate the acute effect of LH on prepubertal male rat serum testosterone within minutes of the intra¬ venous injection of the hormone. Prepubertal male rats of the Holtzman strain arrived in the laboratory at 22 days of age. Five days later, at a body weight range of 68 to 90 g, the rats Were utilized for the following experimental studies: (1) the effect of graded doses of a rat LH preparation on serum testosterone, measured 45 min after a single intravenous injection of the rat LH; (2) the sequence of changes in serum testosterone at 5, 15, 30, 60, 120 and 240 min following a single intravenous injection of rat LH. The rat LH used was a partially purified preparation having a biological potency, according to the OAAD assay (Parlow, 1961), of 0-2xNIH-LH-Sl. The rat LH preparation was dissolved in 0-9% NaCl solution just before use. Doses ofrat LH were administered per 100 g body weight. Intravenous injections

THE SEQUENCE OF CHANGES IN PREPUBERTAL MALE RAT SERUM TESTOSTERONE FOLLOWING INTRAVENOUS INJECTION OF RAT LH