Laboratory rats are considered to be reproductively unresponsive to photoperiod because photoperiod treatments do not induce robust reproductive responses. Groups of 15 young male Fischer 344 (F344) rats were tested for effects of long (16L:8D) and short (8L:16D) photoperiods on testicular development and body mass. Two weeks of short photoperiod inhibited testicular growth, spermatogenesis, and increases in body weight. Testis size became refractory to short photoperiod after 8 wk, but the body weight was lower in short photoperiod for the full 10 wk of the study. In young Harlan Sprague-Dawley rats, in contrast, long and short photoperiod had no effect on either body weight or testis size. Pinealectomized F344 rats had significantly higher body weights and larger testes than did sham-operated controls, suggesting that the effects of photoperiod are mediated, at least in part, by the pineal gland. The F344 strain of laboratory rats is the first in which unmanipulated animals have been found to be robustly affected by photoperiod, indicating that this strain could be a valuable new model for the study of reproductive regulation by photoperiod.