• We attempted to determine whether differences in milk composition or flow rate are the primary determinants in altering breathing pattern during nipple-feeding. In the first phase of the study, 15 neonates were studied during breast-feeding and bottle-feeding; in the second phase, 15 were evaluated during bottle-feeding of expressed human milk and formula. A reduction in inspiratory duration was observed with all feeding regimens. Moreover, significant prolongation in expiratory duration and reduction in breathing frequency were observed during bottle-feeding of formula and expressed human milk (compared with control); these effects were greater with formula feeding. Higher sucking frequency was observed during breast-feeding compared with bottle-feeding; no difference in sucking frequency or sucking pressure was observed between bottle-feeding of expressed human milk and formula. We conclude that most of the differences in sucking and breathing patterns between breast- and bottle-feeding can be attributed to nutrient delivery rather than nutrient composition.