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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Normal and blocked pregnancy were studied in the meadow vole, Microtus pennsylvanicus (Ord), in the laboratory, on individuals bred from wild stock caught near Sudbury, Ontario. The results were then compared with those obtained from wild voles trapped from quadrats during the summers of 1969 and 1970.Criteria were found for distinguishing between individuals in which pregnancy had been blocked and those in which it had not, under laboratory conditions. Nursing females were not found to be susceptible to male-induced blockage and the second set of corpora lutea in the ovaries was not observed to increase the rate of involution of the first set. Females were found to be susceptible to blockage on the 2nd and 5th days after coitus.Some indication of pregnancy failure in the wild was found and the incidence of this phenomonon appears to increase in populations with a higher density. Thus it seems reasonable to hypothesize that pregnancy failure is one of the density-dependent factors that decreases reproduction and ultimately the recruitment of young animals to the population.

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