The fatty acid (FA) composition of neural membranes modulates membrane structure, fluidity and function. Infant dietary FA composition is reported to affect neural tissue FA composition and infant neurodevelopment. We are investigating dietary FA effects upon neurodevelopment in rat pups.Methods: Timed pregnant dams are fed, beginning on day 2 of gestation and throughout lactation, either chow (control) or a purified diet whose fat source (22% of calories) is either corn oil (high n6) or menhaden fish oil (high n3). On postnatal day (pnd) 3, pups are culled to 10 per litter and are randomly cross-fostered among dams of the same diet group to minimize litter effects. Milk is removed from culled pup stomachs for FA analysis. From pnd 4 to pnd 30 pups are assessed daily for the appearance of neurodevelopmental milestones (Smart & Dobbing, 1971). Auditory brainstem conduction times are measured on pnd 23 and 29. Pups are sacrificed on postnatal day 30 and brainstems and cerebrums are removed for FA and morphometric analyses. Results: Ten pups per diet group have been studied. The FA composition of maternal milk, pup brainstems and pup cerebrums reflects maternal diet, with significant enrichment of n3 and n6 FAs in the fish oil and corn oil groups, respectively. The time of appearance of cliff avoidance, auditory startle and visual placing is significantly delayed in pups of fish oil versus corn oil or chow fed dams (p=0.016, 0.001, 0.033 respectively). On pnd 23, auditory brainstem conduction times (msec) are significantly longer in pups of the fish oil group versus corn oil and chow groups (p=0.01), suggesting a slower rate of myelination.Conclusion: Maternal diet supplemented with fish oil throughout pregnancy and lactation is associated with delays in the appearance of several neurodevelopmental milestones and with longer brainstem conduction times in rat pups.Funded in part by Ross Products Division, Abbott Laboratories