Abstract Single smooth muscle cells were isolated from circular muscle of the canine gastric corpus by collagenase incubation. Cytoplasmic pH (pH i) of these cells was measured fluorometrically using the trapped dye 2′,7′-bis(carboxyethyl)-5,6-carboxyfluorescein. Cells were examined for their Na + H + exchange activity after intracellular acidification. Cells acid-loaded by propionate exposure, the NH 4 + prepulse technique or suspension in a Na +-depleted medium regained almost normal pH i upon exposure to a Na + medium. The Na +-dependent alkalinization was amiloride sensitive. As well, addition of amiloride to cells suspended in a Na + medium caused a concurrent decrease in pH i. The study indicates that a Na + H + antiport is present in these smooth muscle cells.