The purpose of this study was to examine the effect of chronic daily hCG treatment on interstitial cell function in the rat, as judged by plasma testosterone levels, the testicular binding of labeled hCG, and the capacity of the testis to respond to gonadotropin stimulation by the production of testosterone in vitro. Twenty-four hours after the first injection of 100 IU hCG there was a significant decline in hCG binding to testis homogenates and an inability to respond to hCG stimulation in vitro. After 7 days of daily injections of 10 IU or 100 IU hCG, the loss of hCG binding was maintained. However, despite the marked decline in hCG binding, there was an enhanced testosterone response to hCG stimulation in vitro, and plasma testosterone levels were significantly elevated. With continued injections of hCG for 14 or 21 days, the testes remained hyperresponsive to hCG stimulation in vitro, but hCG binding returned to control levels, and plasma testosterone concentrations declined and were not statistically different from controls. The latter changes probably result from the formation of specific hCG antibodies (Kd at 4 C, 7.8 ± 4.5 × 10−10 M) that were detected in plasma from rats treated for 14 or more days with hCG. The formation and levels of the hCG antibodies in these animals were sufficient to neutralize the effects of the exogenous hCG, thereby returning plasma testosterone levels to normal and restoring the complement of hCG receptors. (Endocrinology 110: 138, 1982).