The critical day length for testicular response in the tree sparrow ( Passer montanus ). Photosensitive birds were exposed to six different light regimes (9L/15D, 10L/14D, 11L/13D, 12L/12D, 14L/10D and 16L/8D) at fix intensity of light (L = 400 lux; D = 0 lux) for a period of 30 days. Observations were made on testicular volume and body weight at the beginning and end of the experiment and the testicular growth rates (k) were calculated using the formula; K = (lnb-lna)/t where a and b are initial and final testicular volumes, respectively and t is time in days. Significant increase in testicular volume was recorded only when the day length was 11 h/day suggesting that the birds have a critical day length for testicular response above which the rate of growth increases with increasing photoperiods. Birds maintained their normal body weight in all the light regimes showing no significant change.