Abstract Primiparous female hamsters (Mesocricetus auratus) were mated to proven breeders and stressed during early pregnancy. Females were housed singly throughout gestation except for Days 4, 5 and 6 when they were paired for ten-min intervals three times each day with another female matched for age, weight and day of pregnancy. Within each of the pairs, one female was consistently dominant to the other. Controls were exposed to a novel area instead of a conspecific. At parturition, all pups were counted, sexed and weighed. There were no significant differences between control and dominant females' litter sizes or sex ratios (defined as percentage male). Subordinate females produced significantly smaller litters than control or dominant dams and significantly lower sex ratios than control dams. Subordinates produced fewer males than control or dominant dams, but there were no differences in the number of females produced. The paraventricular nucleus (PVN) of the hypothalamus has been implicated as a relay center for the physiological response to stress. Bilateral lesions of the PVN were performed on another group of females and the same protocol described above was followed after the females recovered from surgery. When this 'stress relay center' was lesioned, subordinates did not show the significant deficits in litter size and sex ratio. Sham-operated females showed a similar response to social stress to that of intact females. These results suggest that subordinate dams produce smaller litters via selective resorption of males in utero and that the PVN may be a relay center for the mediation of this response.