In meadow voles, Microtus pennsylvanicus, bred from stock trapped in the wild and maintained under laboratory conditions, a reduction in the incidence of pregnancy (from 60% to 20%) follows exposure of recently inseminated females to strange males of the same species. Bruce (1959) first showed that pregnancy is interrupted if recently inseminated female albino mice, Mus musculus, are exposed to males other than those with which they have mated. In mice, this 'Bruce effect' is known to be due to failure of the fertilized ova to implant in the uterus (Bruce, 1960). Pheromones have been implicated in the mechanism of blockage (Bruce & Parrott, 1960). Male-induced blockage of pregnancy has also been reported in the deer mouse, Peromyscus maniculatus (Eleftheriou, Bronson & Zarrow, 1962; Bronson & Eleftheriou, 1963; Bronson, Eleftheriou & Garick, 1964) and the field vole, Microtus agrestis (Clulow & Clarke, 1968). An experiment was performed with the meadow vole, Microtus pennsylvanicus, to see if the same phenomenon occurs in this ubiquitous North American microtine rodent. Mature virgin voles, bred in this department from stock trapped in the wild, were used in the experiment. From weaning at 20 days of age, the animals were individually housed in plastic cages (28-5 17•5 12•5 cm) with ample water, hay for cover, cotton for bedding, unrestricted oats and carrot twice a

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Female red-backed voles, Clethrionomys gapperi (Vigors), bred from wild stock trapped near Sudbury, Ontario, and maintained at 22 ± 2 °C and 18 h light and 6 h dark in the laboratory, showed no cyclic changes in their vaginal smear patterns. Copulation was followed by the development of corpora lutea in the ovaries, and it is concluded that the animal is an induced ovulator under the conditions of the experiment. The females released 5.1 ± 0.5 ova per ovulation, the gestation period was 18.5 ± 0.5 days, and litter size was 4.6 ± 0.2 pups in the laboratory.