The metamorphic rate of Rana pipiens tadpoles was studied under different photoperiods, daylengths, and feeding schedules. Tail resorption and hindlimb growth and development induced by immersion in 30 pg/l thyroxine (T4) were accelerated under longer photoperiods and continuous light when 6L: 18D, 12L: 12D, 18L: 6D, and 24L regimes were compared. Constant light exposure did not produce faster development than an 18 hr photoperiod, and initially was less effective. The rate of spontaneous and T4-induced metamorphosis was greater the shorter the day on 9L: 9D, 12L: 12D, or 15L: 15D cycles, although all groups received the same overall amount of light, but in different dosages. When feeding schedule but not the LD cycle was varied, groups of tadpoles fed on 18, 24, or 30 hr regimes showed no differences in growth and development rate on 19L: 5D, and only random variations under continuous light. Differences in metamorphic rate on 18, 24, or 30 hr days are not due to the feeding schedules, but to the LD cycles. From these experiments we conclude that illumination, particularly the length and frequency of the photoperiod, affects the utilization of T4. Development rates independent of the total amount of illumination, but related to daylength and light schedule, suggest interaction of light with an endogenous timing mechanism.Light is one of the most important environmental variables that modulates growth and development in the anuran tadpole. However, a stimulatory effect of light is not universal, but apparently depends upon the species.Regimes of 24L and 14L:lOD delayed growth and development in comparison to 6L: 18D, 1L:23D, and 24D schedules in tadpoles of Discoglossus pictus (3, 9 ) . Similar inhibition of growth and development by light occurred in Xenopus Zuevis larvae. In one study, tadpoles showed the fastest rate of development on 24D, followed by 12L: 12D and 24L (4), and in another, Xenopus tadpoles developed faster on 1L:23D than on 23L: 1D (6). It has been suggested that amphibian tadpoles metamorphose when environmental conditions are no longer suitable for aquatic life (3). Perhaps in some cases, long photoperiods generate external conditions, such as warm temperature, favorable to the continued survival of the larvae.On the other hand, early work showed that continuous light promoted metamorphosis in AZytes obstetricans ( 5 ) and Rana temporaria (10). Rana ridibunda tadpoles grew faster on a 6L: 18D cycle than in constant darkness (3). A study of Ranapipiens tadpoles showed faster
