The effect of zinc on galactose and phenylalanine uptake was studied using rat everted jejunal rings. The rings were incubated for 2 min in oxygenated Krebs-Ringer-Tris (KRT) solution. Galactose and phenylalanine uptake was reduced by zinc in a dose-dependent manner, but not in a time-dependent way. One mM Zn2+ but not 0.5 mM Zn2+ inhibited galactose transport without modifying sugar diffusion. Na(+)-dependent phenylalanine transport was reduced by 0.5 mM and 1 mM Zn2+. However, the metal did not change phenylalanine diffusion obtained in the presence of 40 mM L-methionine or Na(2+)-independent phenylalanine transport. Therefore, zinc seems to interact only with the sodium-galactose or sodium-phenylalanine cotransporters. Zinc inhibited sugar and amino acid transport in a non-competitive way, without a significant change in the affinity of the transporters for their substrates and with a Vmax decrease. The inhibitory effect of Zn2+ on galactose and phenylalanine uptake was reversed by washing intestinal rings for 5 min with KRT solution. These results suggest that zinc might exert its inhibitory action by a weak binding to chemical groups related with sodium-substrate cotransporters and located in the luminal membrane of the enterocytes.