Golden hamsters are seasonal breeders in which opiates influence a number of behavioral and endocrine events. After exposure to short days (SD) testes regress, testosterone (T) becomes a more potent inhibitor of luteinizing hormone (LH), prolactin concentrations are decreased regardless of steroid concentrations, and hamsters gain weight. SD exposure also accelerates the loss of copulatory behavior after castration and T replacement is less effective at reinstating copulatory behavior. In hamsters housed in long days (LD) opiates mediate T negative feedback on LH, stimulate prolactin release and inhibit copulatory behavior. In hamsters housed in SD an opiate antagonist delays gonadal regression and opiates no longer affect LH release. Changes in opiate sensitivity may be due to alterations in the distribution or concentration of opiate receptors in specific brain areas. SD decrease $\sp3$H-naloxone binding in the medial amygdala of male golden hamsters. This effect is prevented by pinealectomy or denervation of the pineal. Castration elevates opiate binding in the amygdala, bed nucleus of the stria terminalis and central medial preoptic nucleus regardless of daylength. Maintaining T at long day concentrations prevents increases in $\sp3$H-naloxone binding in hamsters in LD, but hamsters in SD are insensitive to exogenous steroids. T replacement 5 weeks after castration produces binding similar to castrated hamsters in LD or SD. Some functional implications of the opiate binding data were investigated. The medial amygdala is not required for effects of daylength on gonadal regression, T negative feedback on LH, or prolactin. The medial amygdala may participate in the control of body weight. The medial amygdala is required for male sexual behavior. SD increase the sensitivity to methadone inhibition of sexual behavior in castrated hamsters with T maintained at long day concentrations. The opiate binding data suggest that in SD opiate receptors are insensitive to steroidal control. Some of the effects of daylength on sexual behavior and body weight may be mediated through the medial amygdala. The enhanced sensitivity to opiate inhibition of copulatory behavior in SD is correlated with elevated opiate binding in most of the regions measured. These changes are part of the mechanism for daylength actions on copulatory behavior.