The present studies were designed to characterize the developmental aspects of Na(+)-dependent phosphate transport, across the hepatocyte basolateral membranes of the suckling and weanling rats. A well validated technique of plasma membrane vesicles (BLMV) was utilized. Phosphate uptake was driven into an osmotically active intravesicular space as evident by a linear relationship between uptake and 1/osm with no binding component y = 0.04 x-0.03, r2 = 0.99 and y = 0.035x + 0.01, r2 = 0.95 in suckling and weanling BLMV's respectively. The presence of inwardly directed Na+ and pH gradient stimulated phosphate uptake in suckling and weanling BLMV's, however, uptake values under Na+ and pH gradients were greater in weanling rats compared to suckling rats. Kinetics of Na(+)-dependent phosphate uptake were 0.14 +/- 0.01 and 0.28 +/- 0.035 at pH 6.1 (P < 0.05) and 0.1 +/- 0.007 and 0.15 +/- 0.03 nmoles/mg protein/10s (P < 0.05) at pH 7.4 in suckling and weanling rats respectively. Km values were not significantly different. Na+ arsenate and phosphonoformonic acid inhibited Na(+)-dependent phosphate uptake, whereas ATP and P-MB (para-chlomercuribenzoic acid) did not effect Na(+)-dependent phosphate uptake. These studies demonstrate for the first time the presence of a specialized transport system for phosphate across the basolateral membranes of the rat liver during development. This transport system exhibit ontogenic characteristics in regard to its transport capacity.