Six normal adult volunteers received 15 mg/kg of ethambutol (EMB) by mouth, once as an aqueous solution and again as the commercial tablet preparation. Each dose was separated by at least 7 days. Plasma and urine samples were collected at regular intervals for up to 24 and 72 hr, respectively. Peak plasma concentrations ranged from 3.25 to 5.62 mcg/ml, 2 to 4 hr after tablet dosage. Earlier peak times were found after administering the solution. For plasma concentrations up to 12 hr there was a distinct distribution phase followed by an apparent elimination phase with a mean half-life (t 1/2) (+/- SD) of 4.06 +/- 0.53 and 4.78 +/- 0.41 hr for the tablet and the solution, respectively. Excretion rate plots exhibited similar t 1/2 values for the apparent elimination phase. An even longer t 1/2 of approximately 10 hr was evident from 24-hr plasma samples and urinary excretion measurements up to 72 hr. Unchanged drug excreted in the urine averaged 61.1 +/- 3.8% of the dose for the tablet and 63.4 +/- 2.6% for the solution. Plasma protein binding for ethambutol determined by equilibrium dialysis and ultrafiltration was approximately 20% to 30%. The concentration ratio of ethambutol in erythrocytes to plasma ranged from 1.1 to 1.6.