Phosphate transport was studied in brush-border membrane vesicles purified from rat kidney cortex. Influx and efflux were strongly dependent on the presence of cis sodium; the rate of efflux, calculated by linear regression performed on the first time points, was much lower than the rate of influx (0.044 vs. 0.198 pmol.microgram protein-1.s-1). Trans phosphate had a stimulatory effect on phosphate influx (145% stimulation at 10 mM phosphate trans, with 0.2 mM phosphate cis). Trans phosphate was, however, inhibitory for phosphate efflux (89% inhibition at 10 mM phosphate trans). Trans effects of sodium were also studied. With 200 mM trans sodium, we observed 73% inhibition of phosphate influx and 60% inhibition of phosphate efflux. Studies involving sodium and phosphate present at the same time as trans substrates showed that the trans inhibition of phosphate influx by sodium could be completely reversed by trans phosphate. Trans inhibition of phosphate efflux by phosphate was not additive to the inhibition caused by sodium. Addition of trans phosphate had a stimulatory effect on sodium-independent influx, indicating that the binary complex (C-P) could translocate in efflux. These results indicate that the renal phosphate carrier presents a random binding scheme for the intra- and extravesicular sides of the membrane.