Administration of large doses of cobalt-protoporphyrin (CoPP), a synthetic heme analogue, to adult male rats leads to a rapid decline in serum concentrations of testosterone and luteinizing hormone (LH). Hypothalamic luteinizing hormone-releasing hormone (LHRH) contents are not affected by CoPP. The pituitary LH response to exogenous LHRH is attenuated by CoPP both in vivo and in vitro in pituitary cultures. Additionally, CoPP directly inhibits synthesis or release of testosterone by the testes. These effects are specific to CoPP and not shared by heme, inorganic cobalt or tin-protoporphyrin. Castration of CoPP-treated rats leads to a normal rise in LH concentrations, but this can be abolished by using testosterone implants to elevate serum testosterone concentrations to the low levels (- 0.4 ng/ml) typical of CoPP-treated intact rats. Thus, CoPP appears to be a pharmacological model for testing the mechanisms which underlie an enhanced negative feedback efficacy of testosterone on pituitary LH release.