The effects of a low Na medium on PAH transport, renal morphology, and acetate stimulation of PAH transport have been studied. Low Na inhibits PAH uptake and induces vacuolar formation in tubular cells. Both processes are reversible by reincubating in a high Na medium. Vacuolization appears temperature dependent being much greater at 37 than at 25 degrees C. Acetate uptake consists of a passive and an active component, neither of which is sodium dependent. Acetate stimulation of PAH uptake does, however, require Na. This latter observation suggests that Na may act by altering cell metabolism.