Serum concentrations of chloroquine were determined fluorometrically in 100 rheumatoid patients who had been treated with 0.25 gm daily for at least 2 mo. The total dose varied between 3.7 and 400 gm. No patient received more than 75 gm annually. In 15% of the patients side effects were noted. There was a relationship between serum concentrations and side effects but not with the total dose administered. Chloroquine displayed dose-dependent kinetics, which may indicate that close monitoring of serum concentrations is an aid to the safe and rational use of the drug.