Mummichogs Fundulus heteroclitus exhibit changes in the timing of their annual reproductive activity when exposed to experimental photoperiods and temperatures. Although long days and warm temperatures did not maintain gonad maturity past the normal end of the spawning season, premature recrudescence was induced in winter when fish were exposed to 13- or 15-hour light periods (long days) each 24 hours. Interruption of nonstimulatory short days with a brief light pulse during the dark phase was equally effective. This demonstrates that a circadian rhythm of photosensitivity could be responsible for photoperiodic responses. Pretreatment of sexually regressed fish with short days or low temperatures allowed the ovaries to respond to subsequent stimulation by long days and warm temperatures. The combination of short days and low temperatures produced the most rapid gonad response to warm long days. In the spring, fish were capable of spontaneous gonad recrudescence when held in constant darkness at 20 C. Also in the spring, low temperatures appeared to slow or prevent final maturation in the ovary while allowing for significant ovarian growth. Thus, low temperatures may facilitate gonad recrudescence by allowing the preliminary stages of ovarian development to occur while retarding the final maturation of the oocytes.