The effect of different doses of ascorbic acid on the absorptive functions of the gut has been investigated in chicks. The study describes evidence for the existence of a H+/glycine symport system in chick intestinal brush border, because in sodium-free conditions acidification of the apical medium with ascorbic acid causes a dose-dependent stimulation of glycine absorption. Under the above circumstances, potassium acts as a non-essential activator of H+-dependence glycine cotransport. Our results also suggest that glycine uptake is stimulated in a lesser degree by inward sodium gradient. Ascorbic acid stimulation of glycine uptake from K+- free sodium abundant media was diminished, when the dietary level of ascorbic acid was increased to 2000 mg in 1 kg of food. This may result from conformational change in the Na(-)-dependent glycine transporter protein that lowers glycine binding site affinity induced by the over-acidification of the gut lumen. In addition, there was a possible competition between sodium-dependent glycine and ascorbic acid transporters for sodium.