The renal clearance and volume of distribution of gentamicin was measured by the continuous infusion technique in 18 patients with different degrees of renal function. Renal clearance of gentamicin did not differfrom that of inulin. Because 20%–30% of gentamicin in serum is bound to protein, it is likely that some of the antibiotic is secreted by the renal tubules. The mean apparent volume of distribution of gentamicin was 24% of the body weight and was comparable to that of inulin. Twenty additional patients were studied; gentamicin, inulin, and radioactive iothalamate were injected by a single-injection technique. The clearance of these compounds from plasma was slightly greater than their renal clearance, which suggests extrarenal elimination. Confirmatory evidence was seen in the lack of total urinary excretion. The half-life in plasma of all compounds correlated well except in cases of severe renal failure, where half-life of gentamicin was unusually prolonged due to a larger distribution volume. A formula for the calculation of half-life of gentamicin in plasma is given.