Day-night rhythms in feeding behavior and response to the specific kappa opioid agonist U-50,488H (0.10-10. mg/kg) were measured in young (1-2 months), mature (8-12 months) and old (24-30 months) male CF-1 mice. All the mice consumed more food at night than in the day-time, though this nocturnal peak was markedly reduced in old and mature animals. Young mice also displayed a significant, dose-related, nocturnal enhancement in U-50,488H-stimulated feeding. This day-night rhythm was reduced in mature animals and absent in old mice. In old mice, U-50,488H significantly stimulated feeding only after the high dose of 10 mg/kg. Additionally, old animals did not show the dose-dependent latency to initiation of feeding after administration which was observed in young mice and to a lesser extent in mature animals.