The pharmacokinetics of chloroquine were studied after single oral, intramuscular and intravenous administration of the drug in rabbits. Chloroquine concentration was monitored in plasma, whole blood and red blood cells and the pharmacokinetic parameters were determined from the plasma, whole blood and red blood cell concentration-time data. By whatever route the drug was administered, higher concentrations were reached in red blood cells and whole blood than in plasma, but the concentrations declined at the same rate in the three media thus giving similar half-lives. However, the plasma, or whole blood or red blood cell half-lives varied depending upon whether the drug was given orally, intramuscularly or intravenously. With each route of administration the area under the plasma concentration-time curve was less than that for whole blood which was in turn less than that for red blood cells.