Under natural light in Portland, Oreg., captive ring-tailed lemurs (Lemur catta) experience a breeding season that differs by nearly half a year from the season in Madagascar. A series of experimental day length changes from 1971 to 1974 demonstrated the ability of both temperate and tropical photoperiod cycles to induce estrous cycles in quiescent animals. After photoperiodic activation, most impregnated females failed to resume estrous cycles even after infant separations unless they received additional photo-period changes. Unimpregnated females, on the other hand, showed no significant decline in the incidence of estrous cycles under prolonged exposure to a constant day length regimen (12.0L: 12.0D) for over a year.