1. Interrupted-night light schedules were used to look for possible circadian rhythm in photosensitivity in connection with testis regression and recrudescence in the lizard Anolis carolinensis. 2. An hour of lighting during the night of a basic short day length (6 hrs. light/ 18 hrs. dark) in July prevented the normal short-day induction of testis regression in some members of each group but had no effect in others. The average response was independent of the time of night illuminated. 3. In September, a two hour light pulse early in the night had no effect on testis growth, whereas a pulse given late in the night stimulated slight testis growth (a typical long-day response) in 7 of 19 lizards. 4. These findings do not provide a clear demonstration of a circadian mechanism for photoperiod measurement in the lizard and have important implications for the relevance of photoperiod in the ecology of lizards.