Effect of photoperiod on age and BW at attainment of mature semen production was examined in males of lines selected for high (HL) and low (LL) threshold BW at onset of lay, and in males of a commercial Cornish male line (MC), a commercial broiler breeder female line cross (BX), and a commercial layer cross (LX). Photoperiod treatments were: I, increasing natural spring light; and D, steadily decreasing artificial light provided in a light-controlled room. Under I, age at onset of mature semen production was 168.5, 133.6, 138.1, 124.0, and 118.0 d for HL, LL, MC, BX, and LX males, respectively. Under D, age at onset of mature semen production increased by 18.3, 8.0, 55.9, 60.9, and -2.0 d, respectively. The differential effect of Treatment D on the various genetic stocks is attributed to differences between stocks in photoperiodic drive. Therefore, the very great increase in age at onset of mature semen production in BX and MC males points to a severe reduction in photoperiodic drive in males of these stocks, and to future problems in male fertility in broiler breeder flocks entering lay in the summer and fall. Lines HL, LL, BX, and LX rank the same with respect to photoperiodic drive, whether measured in males as in this experiment, or in females, as in a previous experiment. This result suggests that genetic factors affecting photoperiodic drive come to similar expression in males and females. Thus, selection for increased photoperiodic drive in broiler line males may be able to increase photoperiodic drive in broiler line females, and in this way improve female reproductive performance.