Small mammals in the tropics and subtropics usually rely opportunistically on favorable environmental conditions for breeding rather than base their breeding decision on prediction from photoperiodic cues as most high-latitude species do. Species producing precocial young may be more likely to reproduce aseasonally than species with altricial young. For female wild guinea pigs (Cavia aperea) from Argentina (35°S) that produce extremely precocial young, these hypotheses would predict moderate responsiveness of female reproduction to photoperiod. These predictions were tested in a series of laboratory experiments. Guinea pigs reproduced aseasonally when kept under natural photoperiod and temperatures at Bielefeld (52°01′N, 8°32′E). When given short days (9L:15D) and long days (14L:10D) under indoor temperature conditions (20–23°C), no effect of photoperiod on female reproduction was noted. A shift from long day length (14L:10D) to short day length (9L:15D) did not stop reproduction. Increasing energy expenditure for thermoregulation at low temperature (5°C) under long-day (14L:10D) conditions also did not inhibit reproduction. Wild guinea pigs thus reproduce throughout the year without respect to photoperiod as long as food and temperature conditions allow reproduction.