Regulation of LH receptors in the testis is highly sensitive to the circulating LH concentration, and marked loss of receptor sites has been demonstrated after treatment with exogenous gonadotropins such as hCG and ovine LH. Treatment with potent LHRH analogs has also been shown to cause loss of testicular LH receptors in rats, presumably due to elevation of endogenous gonadotropin secretion. To analyze the relationship between the changes in plasma gonadotropins and the receptor content of the testis, the effects of single doses (0.1, 1, and 10 μg) of the natural LHRH decapeptide on LH secretion and testicular receptors were examined in 55-day-old rats. After sc administration of LHRH, serum LH and testosterone values peaked at 15 and 60 min, respectively, and declined to normal values over the subsequent 4 h. In contrast with these rapid and transient plasma hormone responses, the gonadotropin (LH and FSH) and PRL receptor content of the testis exhibited delayed and protracted changes after single doses of LHRH. Between 8–48 h after administration of LHRH, there was a marked and consecutive loss of FSH-, PRL-, and LH-binding sites, each of which returned to control values after about 5 days. The magnitude and duration of receptor loss was related to the dose of LHRH administered, with almost total abolition of binding sites after treatment with 10 μg LHRH. The time sequence of plasma gonadotropin levels in relation to LH receptor regulation was similar to that caused by administration of exogenous gonadotropin, suggesting that the transient peak in serum LH initiated a process of negative regulation - that was manifested by the subsequent decrease in testicular receptor sites. The marked changes in gonadotropin and PRL receptors provide an explanation for the inhibitory effects of LHRH treatment on ovarian and testicular function. In addition, these actions of LHRH can be utilized to investigate the complex process of multiple receptor loss and regeneration that may accompany the physiological actions of pituitary hormones in the testis.