The effects of opioid antagonists on seasonal changes in feeding behavior of white-tailed deer (Odocoileus virginianus) during fall, winter, and summer, as well as in bottle-fed fawns are described. Naltrexone had no effect on the amount of milk ingested by bottle-fed fawns. After weaning naltrexone significantly decreased feeding in both the summer and winter, with no difference in the effect for either season, indicating that opioids do not play a role in the winter hypophagia or the summer hyperphagia in deer. Similarly there was no seasonal effect on the ability of naloxone to decrease feeding in yearlings during summer, fall, and winter. Naltrexone significantly decreased spontaneous locomotion in the fawns in winter and the fall. In yearlings there was a tendency for the high dose of naloxone to decrease spontaneous locomotion in winter whereas in summer and fall it tended to enhance time spent moving. These data suggest that the endogenous opioid control of feeding in deer is independent of the seasonal effect on feeding and endogenous opioids do not play a role in the seasonal rhythmicity of feeding in deer. The effects of endogenous opioids on spontaneous locomotion do appear to be linked to changes in seasonal activity patterns.