Effects of constant photoperiods on seasonality in the male tree sparrow ( Passer montanus ). Photosensitive birds (n = 35) were exposed to light dark cycles of 9L/15D, 12L/12D and 14L/10D close to shortest, equinox and longest day lengths at Shillong for 540 days with light intensity of ~ 400 lux at perch level in light phage and 0 lux in the dark phage. Food and water were provided ad libitum. Periodic changes in testicular volume, bill coloration and body weight were recorded at an interval of 30 days while molt in primary and body feathers were recorded fortnightly during the course of study. Data are presented in means ± s.e.m. Birds showed significant cyclic responses in the above measurements (except in the body weight) only under 12L/12D and 14L/10D while those under 9L/15D failed to do so suggesting the existence of photoperiodic mechanism in their control and exclude the possibility of circannual rhythm generation. Body weight did not appear to be photoperiodically regulated.