Laboratory rats have been generally considered non-photoresponsive, but strains of laboratory rats have been found to be variable for this trait. Young males of both the Fischer (F344) and Brown Norway strains (BN) suppress reproductive development, food intake and body mass in short winter photoperiod (short days (SD); 8 h light:16 h darkness), and food restriction interacts with SD to enhance the effect of SD alone. Conversely, young male Harlan Sprague Dawley outbred rats, along with other outbred laboratory rats tested, have little or no response to SD except when unmasked by food restriction or other treatments, and have generally been considered nonphotoperiodic. In order to assess how widespread this trait might be among rat strains, and to test for uncoupling of reproductive and nonreproductive responses, we tested 3 additional inbred strains, including ACI, PVG and BUF rats, for photoresponsiveness and for unmasking of photoperiodic responses by food restriction. Young males of all three inbred strains exhibited photoresponsiveness in testis mass (5 -20% lower in SD), seminal vesicle mass (20-50% lower in SD), and body mass (5-10% lower in SD). Food restriction also suppressed reproduction, but there was little or no interaction with the effects of photoperiod. The results are consistent with the hypothesis that laboratory rats are genetically variable for photoperiodism, and that photoresponsiveness may be widespread among inbred rat strains, as all five inbred strains tested have shown photoperiodic responses. The results are particularly important because standard research protocols may unknowingly manipulate this pathway in rats, causing unsuspected variability among or within studies.