Rattus norvegicus has been considered nonphotoperiodic, but Fischer 344 (F344) rats are inhibited in growth and reproductive development by short photoperiod (SD). We tested photoresponsiveness of the genetically divergent Brown Norway (BN) strain of rats. Peripubertal males were tested in long photoperiod or SD, with or without 30% food reduction. Young males were photoresponsive, with reductions in testis size, body mass, and food intake in SD and with enhanced responses to SD when food restricted. Photoperiods 11 h of light inhibited reproductive maturation and somatic growth, whereas photoperiods of 12 h or more produced little or no response. F344/BN hybrids differ from both parent strains in the timing, amplitude, and critical photoperiod of photoperiodic responses, indicating genetic differences in photoperiodism between these strains. This is consistent with the hypothesis that ancestors of laboratory rats were genetically variable for photoperiodism and that different combinations of alleles for photoperiodism have been fixed in different strains of rats.