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The effects of photoperiod in stimulating reproductive function in prepubertal boars was studied in 40 crossbred boars. One group of boars was exposed to a regimen where day length was increased from 12 to 14.5 h/d (from 8 to 20 weeks of age) and then decreased from 14.5 to 12 h/d (from 20 to 32 weeks of age); whereas, the other group of boars was exposed to a regimen where day length was decreased from 12 to 9.5 h/d and then increased from 9.5 to 12 h/d. Exposing prepubertal boars to a long photoperiod inhibited the development of the testis at 24 weeks of age. The inhibitory effect of long days on testis development at 24 weeks of age was overcome by decreasing the photoperiod. Short days reduced the level of sexual behavior at 25 and 26 weeks of age. The inhibitory effects of short days on sexual behavior was overcome by exposing boars to a longer period. This study implies that young boars reared during short days may need to be exposed to a longer photoperiod before expressing an adequate level of sexual behavior and young boars reared during long days may need to be exposed to shorter days to increase their sperm production capability.