Day-night rhythms in feeding behavior and response to the putative sigma opiate agonist, N-allylnormetazocine (+/- SKF-10,047, 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. The mice consumed more food at night than in the day-time, though this nocturnal peak was markedly reduced in the mature and old animals. The young mice also displayed a significant nocturnal enhancement in SKF-10,047 (0.10-1.0 mg/kg) stimulated feeding, that could, in part, be suppressed by the opiate antagonist naloxone (1.0 mg/kg). The day-night rhythm in ingestive responses to SKF-10,047 (0.10-1.0 mg/kg) was reduced in the mature animals and absent in the old animals. The old mice failed to show any significant increase in ingestive response following opiate administration. A higher dose of SKF-10,047 (10 mg/kg) had no significant ingestive effects in any of the age groups of mice; the excitatory, psychotomimetic-related effects, being also reduced in the old animals.