Long-term recordings of locomotor activity, feeding activity and core temperature carried out in 7 male and 7 female adult owl monkeys (Aotus lemurinus griseimembra) revealed sex-specific infradian alterations in the level of these circadian functions when the monkeys were housed under lighting conditions which neither inhibited nor enhanced (i.e., "masked,") their circadian activity rhythms. Such nonmasking lighting conditions were: constant dim light (LL) at 0.1-0.5 lx, photoperiods consisting of 0.5 h light (L) at 80 lx and 23.5 h darkness (D) at 0.5 lx, and skeleton photoperiods consisting of two 80-lux light pulses of 0.5 h applied at intervals of 12:12 h and 9:15 h, respectively. In 5 of the female and none of the male owl monkeys, the amount of locomotor activity per day or per circadian cycle, increased at an average interval of 14.0 +/- 2.3 days to 198.4 +/- 48.2%, while the feeding activity was concomitantly reduced to 53.7 +/- 11.2%, and the core temperature level dropped by 0.3 +/- 0.1 degrees C, as compared to the respective preceding level of these parameters. The period of this infradian periodicity superimposed on the circadian rhythms corresponds approximately to the ovarian cycle length of Aotus (6,7). As food deprivation for one day resulted in a drop in core temperature is probably the result of reduced food intake. The infradian rise in activity and the decrease in food intake might be an effect of the ovarian cyclic variation in estrogen level.