Five elderly achlorhydric patients and five controls took 250 mg tetracycline as a capsule or as an oral solution. Venous blood samples drawn during the 12-hr period following administration and urine collected for 72 hr post-dosing were assayed fluorometrically for tetracycline. There were no differences between the two groups in either rate or extent of tetracycline absorption. The mean apparent first-order rate constant (ka) for absorption of drug from capsules was 0.58 +/- 0.17 hr-1 for patients and 0.65 +/- 0.21 hr-1 for controls. The mean extent of absorption for capsule dosage forms relative to an oral solution was 1.0 +/- 0.20 for patients and 1.0 +/- 0.24 for controls. Concurrent administration of 2 gm of sodium bicarbonate to 2 of the normal subjects resulted in no impairment of either the rate or extent of tetracycline absorption from capsules, suggesting that the effect of elevated gastric pH on tetracycline bioavailability may relate to formulation.