Activity rhythms of 3 species of day‐active sciurid rodents under various constant lighting conditions and in L/D schedules were studied. The animals included 8 white‐tailed antelope squirrelAmmospermophilus leucurus,5 black‐tailed antelope squirrelA. harrisi,and 5 eastern chipmunkTamias striatus.All species showed free‐running rhythms in constant conditions, with the period decreasing at higher intensities. The 3 species synchronized readily to a variety of light schedules, with the exact response dependent upon the phase relationship of activity to the new light schedule.