Phosphate transport was investigated in brush-border membrane vesicles prepared from the duodenal segment of the chicken small intestine. Phosphate transport was sodium dependent and demonstrated the typical overshoot phenomenon with an external sodium gradient. There was no appreciable difference in phosphate uptake rates at pH 6.5 or 7.5 values, and the apparent kinetic parameters suggested that both the monovalent form, H2PO4-, and divalent form, HPO4(2-), were transported at the appropriate pH values. The apparent Km values were 0.10 and 0.07 mM, respectively. Determination of the sodium stoichiometry, assessed by an external sodium concentration or by the static-head method, suggested an interaction of one or more sodium ions with phosphate at the acid pH value and two or more sodium ions with phosphate at the alkaline pH value. The sodium affinity for sodium-dependent phosphate transport was in the order of 10-25 mM and was similar at both pH values. Phosphate accumulation was not affected by an externally applied potassium diffusion potential, suggesting an electroneutral process at both pH values. The cumulative evidence would suggest Na+-H2PO4- transport at pH 6.5 and 2Na+-HPO4(2-) at pH 7.5 value, and the effect of hydrogen ions on sodium-dependent phosphate transport is dependent on the external sodium concentration. Brush-border membranes isolated from chickens maintained for 5-7 days on a low-phosphate diet demonstrated a greater sodium-dependent phosphate uptake (Vmax) than animals on normal phosphate intakes with no change in the apparent Km. Sodium stoichiometry and the rheogenic nature of the process appear to be similar to those present in normal birds.(ABSTRACT TRUNCATED AT 250 WORDS)