Goldfish Carassius auratus held on a 14L:10D (14-hour-light:10-hour-dark) photoperiod and fed once daily at unscheduled times during light and darkness had a diurnal activity pattern beginning about light onset; this rhythm lasted at least 3 days when fish subsequently were held in constant light or constant darkness. When goldfish were fed once daily at scheduled times during a 12L:12D photoperiod (the schedule varying among treatments with respect to light and darkness), their activity was entrained by the feeding time, and the activity cycle persisted through 3–10 days of starvation. Serum-cortisol concentrations also were entrained by feeding regimen, peaking 0–6 (often 4) hours before feeding, and mean daily titers were unaffected by the time of day feeding occurred. Serum-thyroxine concentrations, in contrast, were entrained by photoperiod, though daily mean titers were affected by feeding time. Thus, feeding schedule, when it provides a consistent cue, overrides photoperiod to entrain cyclic goldfish activity and serum-cortisol variations–but not serum-thyroxine variations. This implicates a multioscillatory system in the temporal integration of goldfish with their environment. For study of some endocrine rhythms in fish, activity cycles may provide a better reference than photoperiod.