There is sexual activity in every month of the year, but its incidence varies greatly from one season to another. As has been indicated by various authors, male voles reach sexual maturity later than females. It seems that ordinarily the availability of sexually active males is not a limiting factor, however. While males that are still well below average adult size produce mature spermatozoa, and are probably capable of breeding (Jameson, 1947: 145), certain large old males may sire a disproportionately large percentage of the litters produced. Observations on males in confinement indicated that sexual activity tended to be directly proportional to the size of the testes. Occasional individuals, having much enlarged scrotal testes were more readily stimulated to sexual activity and more aggressive toward females than were those in which the testes were of more nearly typical size or abdominal or were smaller than normal. The combination of factors controlling size of testes is not well understood, but males having unusually large testes were caught most often when food supply was optimum, for instance after a period of heavy precipitation when an abundant supply of new grass provided succulent and nutritious food. In confinement sexual activity was largely inhibited and attempts to establish a laboratory colony met with failure. Sexual activity was observed mainly in recently captured males, and their interest was aroused chiefly by females that had given birth to litters within a few hours previously. Oestrus is known to follow closely after parturition. Females found in live-traps with newborn young often were brought to the laboratory for observation. An apparent instance of hostility between rival males competing for an oestrus female was observed on September 2, 1950. The female was found in a trap with four newborn young, and since the young had not yet attached to her teats, she was temporarily returned to the trap after recording, to prevent desertion of the litter. Returning twenty minutes later I found another adult vole at this trap. It would suddenly emerge from dense grass nearby, and would move over the trap or around it, with jerky, halting movements, then would dart back under cover. The female emerged from the nest box into the trap runway, and sniffed at the other, and both pressed against the intervening wire barrier. There was gnawing on the wire by one or both. A third adult vole appeared. As it moved toward the trap, all three suddenly took alarm and darted back under cover, the female hiding in the trap nest box. In a few seconds they again appeared. The two outsiders, presumably both males, were not individually recognizable, but several times one was seen to dart at the other, chasing it away momentarily. They were seldom both in sight at once.