SummaryLittle information is available regarding the time, rhythm, number, and appropriate dosage of human chorionic gonadotropin (HCG) for adequate testing of testicular function in human. The time course of the effect of two, three, or seven HCG injections at intervals of one, five, and two days, respectively, on the plasma levels of testosterone was studied in 11 boys. The first injection induced a progressive and modest rise of T. The second given one day later had little additive effect, maximal values being seen 72 to 120 hr later. In the prepubertal boys to whom several HCG injections were given, testosterone levels reached comparable levels after four injections every five days or seven injections every other day. Although the number of subjects studied was relatively small, these results give some rational basis for the following HCG test: two or four injections at four-day intervals.SpeculationIn adult men or rats, human chorionic gonadotropin induces a rapid increase in testosterone biosynthesis, followed by a steroidogenic desensitization phenomenon and a trophic effect. The recruitment of new functioning Leydig cells probably occurs parallel to the steroidogenic refractoriness of the cells when gonadotropic stimulation reaches a certain level.During pubertal development, luteinizing hormone also induces the differentiation of existing nonsteroidogenic interstitial cells into Leydig cells. Meanwhile, circulating levels of testosterone progressively increase. Assuming that endogenous luteinizing hormone has the same actions as human chorionic gonadotropin, it is speculated that if a possible induction of refractoriness of the Leydig cells is present at this stage, it might “modulate” the testicular response to the increasing levels of luteinizing hormone and partially explain the relatively slow achievement of a fully active testicular secretion in humans.