The effect of mature males on the sexual development of young female and male field voles, reared in either long (stimulating) or short (inhibiting) photoperiods, was examined. Females reared in the presence of a mature male had a more advanced state of sexual maturation (as indicated by uterine weight) than did females reared in isolation from males, in long and short photoperiods (P < 0\m=.\01). No interaction between photoperiod and male presence was found. Augmented uterine growth occurred

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... ot only when young females were separated from mature males by a wire mesh or solid metal screen but also when they were merely exposed to bedding previously used by mature males. Castrated males had no effect on the sexual development of females. The effect ofmature males on the sexual development of young males was less clear, although there was some indication that the presence of adult males inhibited their sexual development in long and short photoperiods. For males and females, growth rate was stimulated by long photoperiod, but no effect of male presence on growth rate was found. Materials and Methods Animals. All animals were from the field vole colony maintained in the Department of Agricultural Science, Oxford, on a photoperiodic regimen of 16 h light:8 h dark (16L:8D) (Baker & Clarke, 1986). They were fed oats and Dixon's diet 41 ad libitum, supplemented twice weekly by carrots and hay. Water was constantly available. The experimental animals, which were weanlings * Present address: