The present work has been undertaken to obtain a direct evidence of the involvement of chemical gradient of Na+ in organic acid transport in the renal tubules. Superficial tubules of rat and turtle kidneys were depleted of Na and K during cold preincubation to be then incubated in the presence of ouabain under anoxic conditions at 30 degrees C. Uptake of an organic acid, fluorescein, was detected in proximal tubules of the essential surface of kidneys by means of contact microfluorometry. The creation of artificial gradient of NaCl (medium to cells) stimulated the uphill uptake of fluorescein in the tubules. The dissipation of Na gradient was accompanied with the efflux of fluorescein from tubules. The presence in the medium of harmaline, an inhibitor of Na-dependent transport systems, as well as the replacement of Cl- for SO2- or SCN- resulted in the disappearance of the stimulatory effect of Na gradient on the fluorescein transport. It is concluded that the fluorescein transfer across the baso-lateral membrane of tubular cells may be energized by Na chemical gradient, and this process significantly depends on the anionic environment.