Abstract Exposure to short, winter-like daylengths produces deficits in male golden hamster copulatory behavior, even when exogenous testosterone is administered to replicate serum concentrations typical of hamsters housed in long days. Daylength also regulates opiate receptor concentrations in limbic brain regions which control sexual behavior, and the response of gonadotropin secretion to opiate receptor antagonists is modulated by daylength. This study tests the hypothesis that short days amplify the opiatergic inhibition of copulatory behavior. Male golden hamsters were castrated and given testosterone implants before transfer to short days. Hamsters were tested for copulatory behavior after injections of saline or various doses of methadone, an opiate agonist believed to be specific for mu receptors. Locomotor activity was also measured. Hamsters housed in short days showed copulatory deficits in response to lower doses of methadone than hamsters housed in long days. Short days enhanced copulatory deficits after methadone at doses which did not affect general activity.