Throughout pregnancy, Lewis rats were fed standard rat chow supplemented with 15% (w/w) of either MaxEPA fish oil (FO) or arachis oil (AO); a third group was fed standard rat chow only (St) (n = 15, 15, and 16 rats, respectively). Compared to AO-rats, FO-rats had substantially higher levels of n-3 fatty acids and lower levels of n-6 fatty acids in maternal and fetal tissues at delivery. FO-rats had a longer average gestational age than AO-rats (p less than 0.01), primarily due to a high proportion of AO-rats with short gestations. Birthweights of FO-rats were lower than those of AO-rats (p less than 0.05), which may be related to the lower maternal weight gain during pregnancy and the lower food consumption observed in FO-rats (p less than 0.01). Litter size and occurrence of stillborn pups were not significantly different in the groups (p greater than 0.1). The findings are compatible with the hypothesis that gestational age is positively related to a high dietary intake of n-3 fatty acids.