The effect of photoperiod on the rhythm in pineal N-acetyltransferase activity and on the growth of testes and of brown adipose tissue was studied in infant rats at the age of 5–15 days. After exposure to light at night, high N-acetyltransferase activity declined in 6-, 7-, and 8-day-old intact rats, but not in 5-day-old intact rats or in 8-day-old blinded rats. Hence, light might penetrate under closed eyelids and might be perceived by retinal photoreceptors in rats older than 6 days. In 15-day-old animals, the amplitude of the N-acetyltransferase rhythm was ten times higher than in 5-day-old rats. The pattern of the N-acetyltransferase rhythm, namely the duration of elevated nocturnal activity, was different under long and short photoperiod at least 1 week before opening of eyes. Photoperiod affected the growth of testes and of interscapular brown adipose tissue already in the preweaning period. In 15-day-old rats, immediately after opening of eyes, the weight of testes was significantly higher and the weight of brown fat lower under long than under short photoperiod. Photoperiodic regulation thus occurs before rats open their eyelid