Plasma corticosterone levels of female lizards (Anolis carolinensis) were sampled at 4-h intervals before and after a photothermal regimen known to stimulate ovarian growth in January and February, respectively. Female A. carolinensis have a diurnal cycle in plasma corticosterone with a daily peak at 1700 h in lizards with unstimulated ovaries exposed to short photophase and low temperature (12L 20°C:12D 20°C). Stimulation of ovarian growth by increasing the temperature and lengthening the photophase (14L 32°C:10D 20°C; 80% relatively humidity) caused an 8-hr shift in the phase of the corticosterone peak to 0900 h. In lizards exposed to stimulatory temperature and photoperiod but also to low humidity (<35%) corticosterone exhibited a biphasic rhythm, with peaks at both 0900 and 1700 h. These results suggest that conditions which stimulate reproductive activity also cause a phase shift in the daily rhythm of plasma corticosterone. The results also suggest that chronic low humidity during these stimulatory conditions creates a biphasic rhythm of plasma corticosterone in this species. Therefore, a natural condition, low humidity, may be a chronic stressor of the lizard Anolis carolinensis.