The effects of immunoneutralization of endogenous gonadotrophin releasing hormone (GnRH) on the serum concentrations of testosterone and gonadotrophins and the binding of 125I-labelled human chorionic gonadotrophin (HCG) to testicular membrane fractions were studied in adult male rats. Four days after the administration of 1 ml anti-GnRH serum, the level of testosterone in the serum decreased to 44% of the concentration before the injection, whereas administration of normal rabbit serum had no effect. Multiple injections of anti-GnRH serum for 4 days dramatically suppressed the secretion of gonadotrophins in rats orchidectomized 2 months earlier. In intact male rats treated identically, immunoneutralization of GnRH decreased the level of serum testosterone to 32% of the concentration present in saline-treated controls, but did not decrease the number of testicular binding sites for HCG (LH). Administration of testosterone or oestradiol for 3 or 6 days caused a marked reduction in the concentration of serum gonadotrophins but did not decrease the number of LH receptors. This study provides further support for the concept that one releasing hormone governs secretion of both FSH and LH. In addition, these studies indicate that selective reduction of gonadotrophins for 3-6 days has no effect on the number of testicular LH receptors. This suggests that pituitary hormones other than gonadotrophins may be important in the maintenance of testicular receptors for LH.