There have been many trials describing the effects of polyunsaturated fatty acids (PUFA) on fecundity, neonatal development, and maternal behavior in humans but few controlled studies in rodents. We examined the effects of a maternal diet high in N-3 or N-6 PUFA on NIH Swiss mice. Female mice were ad libitum fed one of three complete and balanced diets (N-3, enriched in menhaden oil; N-6 enriched in corn oil; C, control diet, Purina 5015) from age of four weeks until end of the study. Mice were bred at ~19 weeks and 27 weeks of age, providing a total of 838 pups from 129 litters in two experiments. After weaning their pups from parity 1, behavior of dams was assessed on elevated-plus and open field mazes. While fraction of male pups from N-3 and C groups was not different from 0.5, dams on N-6 diet birthed more daughters than sons (213 vs. 133; P < 0.001). Although maternal stress has been reported to favor birth of daughters, behavior of N-6 dams was not different from controls. By contrast, N-3 dams displayed greater anxiety, spending less time in open arms and more time in closed arms of the elevated maze, and traveling less distance and exhibiting less exploratory behaviors in the open field (P < 0.05). N-3 dams tended to produce smaller litters than C dams, and N-3 suckled pups gained less weight (P < 0.05). In conclusion, N-3 diet had negative effects on murine fecudity and maternal behavior, while N-6 diet favored birth of daughters.