Arachidonic acid (ARA) is a long chain polyunsaturated fatty acid that is routinely added to infant formula with docosahexaenoic acid (DHA) to support neonatal growth and development. Optimal levels of ARA in formula remain to be determined and are based largely on mean worldwide values in breast milk. The objective of this dissertation research was to determine the role of dietary ARA in neonatal development and to evaluate the optimal level in formula to support proper growth and development. Two studies with neonatal domestic pigs were carried out to achieve this objective. The purpose of the first study was to determine the effect of the dietary ARA level on growth, clinical chemistry and immune function, and on tissue ARA and DHA accretion. On day 3 of age, formula-reared (FR) piglets were assigned to 1 of 6 milk replacer formulas containing ARA/DHA as follows (% fatty acid (FA)/FA): (a1) 0.1/1.0; (a2) 0.53/1.0; (a3-d3) 0.69/1.0; (a4) 1.1/1.0; (d2) 0.67/0.62; (d1) 0.66/0.33. No significant differences were observed among the FR groups for growth, hemogram, clinical chemistry or measures of immune status. Heart and liver ARA were responsive to dietary ARA while brain and retina were not. Heart ARA was particularly sensitive to dietary ARA. In the second study, we evaluated two ARA oils for potential use in infant formula compared with a third, commercially available ARA oil. Diets were fed on days 3 - 22 of life and supplied ARA at 0.64% FA and DHA at 0.34% FA. We observed no toxicological effects or differences in growth, and concluded that the experimental ARA oils are safe and bioequivalent to the commercially available ARA source. Overall, results from this dissertation research suggest that the dietary ARA level has a negligible effect on growth and development of the central nervous and immune systems when DHA is supplied near the high end of human breast milk levels (1% FA). The unique responsiveness of the heart to dietary ARA level revealed that ARA accretion is limited by dietary supply. Further investigations are warranted to determine the significance of ARA status on immediate and long term cardiac physiology.