(1) Long-day photoperiod (20L:4D) and cold (5°C) temperature (LC); (2) long-day photoperiod and warm (25°C) temperature (LW); (3) short-day photoperiod (4L:20D) and cold temperature (SC); and (4) short-day photoperiod and warm temperature (SW). A fifth group, kept at a constant temperature of 20°C and 10L:14D photoperiod, was injected with 100 IU chorionic gonadotropin daily, for 10 days. The controls were freshly trapped animals. At the end of the experimental period the animals were killed and the testes processed and examined histologically.

The results showed that, whereas the control animals had quiescent gonads, all the experimental animals, regardless of light and temperature regimen, presented active spermatogenesis. Only quantitative, histological differences were found between the various groups. The most active testes were found in the HCG-treated group, followed by groups: LC, SC, LW, and SW, in this, decreasing, order.

It was concluded that a combination of long-day photoperiod and low temperature has a stimulatory effect upon the testes of red-back voles and that mating behavior in the male vole appears not to be directly correlated with the degree of testicular development and spermatogenesis.

The possible role of nutrition and stress upon gonad activation is discussed.