The effect of feeding on calf blood flow (CBF) was investigated in 27 preterm and 10 term small-for-gestational-age (SGA) neonates using the venous occlusion plethysmographic method pre- and postprandially. Blood pressure and heart rate were simultaneously monitored. Feeding consisted of commercially prepared formula with average amounts of 20 ml/kg for preterm and 30 ml/kg for term SGA infants. As a group, the preterm neonates showed no significant CBF changes 1/2 h postprandially from the control value of 8.4 +/- 2.6 ml/min/100 ml (mean +/- SD). Although the difference was not statistically significant, 10 of the 27 preterm infants showed a mean CBF decrease of 9% and 17 showed no change to a slight increase immediately postprandially. These changes were followed by a 35% mean increment in CBF of all preterm infants at 2-2 1/2 h postprandially. CBF decreased in term SGA infants by 40% at 1/2 h postprandially from a control flow of 8.6 +/- 2.9 ml/min/100 ml (p less than 0.01). CBF returned to the control level 1-1 1/2 h postprandially and superseded the control value by 16% at 2-2 1/2 h. Peripheral vascular resistance varied inversely with CBF. Blood pressure and heart rate showed no significant changes. These observations indicated that unlike term infants, most preterm infants manifest no significant immediate peripheral vascular response to feeding before the onset of a hyperdynamic circulatory state. This difference was related to gestational age rather than to birth weight as term SGA infants responded like term appropriate-for-gestational-age infants. The possible mechanisms involved in these CBF changes with feeding are discussed.