A brain site of melatonin action has been determined for the white-footed mouse (Peromyscus leucopus). Melatonin-beeswax implants releasing small quantities of melatonin (<100 ng/day) caused a 50% reduction in reproductive tract weight relative to controls (p < 0.025) with 83% of these animals having an imperforate vagina, when implanted in the anterior hypothalamic nuclei (AH) and suprachiasmatic nuclei (SCN). Subcutaneous implants had little effect. Implants in the AH and SCN also had a pronounced effect on both lipid-free interscapular brown fat and nesting behavior. Mice implanted in these regions exhibited a 59% increase in interscapular brown fat and 65% more nesting than controls (both p < 0.01). These results suggest that melatonin acts at a region in the anterior hypothalamus which controls photoperiodic adjustments.