Acute single overdosage (1.0 mg/kg, i.m.) with the novel benzmorphan narcotic analgesic agent Cogazocine lactate lowered serum LH and testosterone concentrations of Beagle dogs. The effect was rapidly reversible and persisted for less than 24 h after drug administration. Repeated administration (1.0 mg/kg, i.m.) induced reversible suppression of spermatid maturation with subsequent effects on semen characteristics. Examination of circulating LH and testosterone concentrations, together with pituitary and prostate morphology, suggested that the changes were due to inhibition of LH release. It is probable that the underlying mechanism is an exaggerated pharmacological response to repeated overdosage, common to many potent narcotic analgesic agents.