The age at which photosexual responsiveness occurs in juvenile male turkeys was investigated by determining the age at which a postcastration rise in serum luteinizing hormone (LH) was first evident. Birds were subjected to short (6L:18D) and long (16L:8D) photoperiods. In birds exposed to long photoperiod there was no castration-induced LH rise in 2-, 4-, and 6-week-old turkeys, but there was in such birds by 7-8 weeks. No postcastration increase occurred if castrated turkeys were held under 6L:18D. When 10-week-old castrated turkeys were switched from a long to a short photoperiod their serum LH levels fell to that of intact controls. Administration of synthetic luteinizing hormone-releasing hormone (LHRH) to 6-week-old turkeys increased serum LH about ninefold above the preinjection values. Hypothalamic LHRH content of 6- and 10-week-old males were similar. Thus, it is proposed that the absence of LH response to castration in these young birds is due to a failure to transduce or transmit the photoperiodic information necessary to activate the hypothalamic-pituitary axis.