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Effect of photoperiod and melatonin on testicular development in the white-footed mouse, Peromyscus leucopus.

At autopsy the absolute and relative testicular weights of 6-week-old white-footed mice were significantly lower (P < 0.001) in animals exposed to a short photoperiod (8L : 16D) or exposed to a long photoperiod (16L : 8D) and treated with a 1 mg melatonin s.c. implant than in those kept only in a long photoperiod. The long photoperiod animals had tests of normal appearance and complete spermatogenesis, but there had been little development in the testes of the other animals and seminiferous tubule diameters were reduced.