Dietary n-6 or n-3 fatty acid deficiencies result in changes in brain and retinal phospholipid composition that can affect cell membrane and organ function. An n-3 fatty acid deficiency has been associated with altered electroretinograms and reduced visual acuity in animals. Other promising methods for assessing the effects of fatty acid deficiencies on brain and retinal maturation include visual-evoked potential acuity, sleep-wake cycle, auditory brain stem-evoked response, somatosensory-evoked potential measurements, and the Fagan and forced-choice preferential looking acuity tests. Preterm infants fed a formula low in alpha-linolenic acid (18:3n-3) had significant electroretinographic changes at discharge from the nursery, indicating a delay in rod photoreceptor maturation. However, infants fed human milk or supplementary n-3 fatty acids as marine oil had electroretinogram indexes like those of infants of comparable age tested soon after birth. Visual cortex function, measured by pattern reversal visual-evoked potential and forced-choice preferential looking visual acuity response, was also better in infants fed human milk or marine oil-supplemented formula than in infants fed formulas without docosahexaenoic acid (22:6n-3). Studies of term infants suggest that visual acuity is more mature in breast-fed than in formula-fed infants [corrected] at 4 months and 3 years of age.