Our aim was to obtain a better understanding of the differences between breast-feeding and bottle-feeding, particularly with regard to how sucking performance changes from nonnutritive sucking (NNS) to nutritive sucking (NS). Twenty-two normal term infants were studied while breast-feeding at 4 and 5 d postpartum. Five of the 22 infants were exclusively breast-fed, but we tested the other 17 infants while breast-feeding and while bottle-feeding. Before the milk ejection reflex (MER) occurs, little milk is available. As such, infants perform NNS before MER. For bottle-feeding, a one-way valve was affixed between the teat and the bottle so that the infants needed to perform NNS until milk flowed into the teat chamber. At the breast, the sucking pressure (-93.1 +/- 28.3 mm Hg) was higher during NNS compared with NS (-77.3 +/- 27.0 mm Hg). With a bottle, the sucking pressure was lower during NNS (-27.5 +/- 11.2 mm Hg) compared with NS (-87.5 +/- 28.5 mm Hg). Sucking frequency was higher and sucking duration was shorter during NNS compared with that during NS both at the breast and with a bottle. There were significant differences in the changes of sucking pressure and duration from NNS to NS between breast- and bottle-feeding. The change in sucking pressure and duration from NNS to NS differed between breast-feeding and bottle-feeding. Even with a modified bottle and teats, bottle-feeding differs from breast-feeding.