Technical Abstract: Little is known about how early postnatal diet affects brain processes related to cognitive function in healthy infants. To address this question we examined EEG activity recorded from 3 month old infants [breastfed (BF: n = 104, 55 males), milk-based formula fed (MF: n = 114, 57 males) or soy formula fed (SF: n = 108, 55 males)] as they were being presented two syllables.(oddball paradigm). Time-frequency analyses were conducted on EEG (.5-12 Hz in 2 Hz bands) recorded over temporal brain regions during the first 250 msec following syllable presentation. Data were analyzed using ANOVAs and post-hoc t-tests. Group differences in post-stimulus power [BF > MF (across bands, all p = .05); BF > SF (10-12 Hz, p < .05) largely reflected gender influences, i.e., Males: BF > MF, all bands, p = .05; SF > MF, .5-6 Hz, p = .05); BF > SF,10-12 Hz, p = .05; Females: all across-group and frequency comparisons of response power were non-significant. Only MF infants showed gender differences in power (females > males: 4 Hz and above, all p = .05). These findings show that: early infant diets differentially influence the development of EEG neural networks engaged during initial acoustic and phonemic evaluation of language sounds; these networks are least-well developed in MF males; among males in formula groups, network development was greater in SF than MF infants; and, no diet effects were observed in females.