Physicochemical considerations suggest that citrate may potentiate gastrointestinal aluminum absorption via the formation of an aluminum citrate moiety. We tested this hypothesis and also studied whether sodium bicarbonate would have a similar effect. Eight healthy adults each partook of four oral regimens: aluminum alone, aluminum plus sodium bicarbonate, aluminum plus citrate (as Shohl's solution), and citrate alone. Twenty-four hour urine collections were obtained immediately preceding and during the second day of each medication period for determination of aluminum content. A significant but similar increment in urinary aluminum occurred with both aluminum alone and aluminum plus sodium bicarbonate, while only a small increment was noted with Shohl's solution alone. The rise in urinary aluminum obtained with aluminum plus Shohl's solution, however, was nearly eight times that seen with either aluminum alone or aluminum plus sodium bicarbonate (327 micrograms vs 45 micrograms and 41 micrograms, respectively). Citrate thus appears to augment gastrointestinal aluminum absorption markedly, an effect not shared by an equivalent dose of sodium bicarbonate. Citrate administration to patients with renal failure who are also taking aluminum-containing medication may be harmful.