1. Chloroquine was given by a single intravenous injection to rabbits and to two groups of patients. The concentrations of the drug and its more polar metabolites in plasma and tissue were measured by a fluorometric method.2. The concentrations of chloroquine in rabbit liver exceeded those in skin and these in turn exceeded those in plasma. Chloroquine concentrations in rabbit skin were fairly steady between 24 and 72 h after injection. Chloroquine was still present in skin 30 days after the injection. Metabolites of chloroquine were present in plasma, skin and liver.3. Skin was taken from patients 48 h after an intravenous dose of chloroquine (1.25 mg/kg). Skin from patients prone to chloroquine-induced pruritus contained higher concentrations of chloroquine (P<0.01) and lower concentrations of metabolites (P<0.01) than skin from other patients.4. It is suggested that increased liability to chloroquine-induced pruritus may be associated with a lower rate of chloroquine metabolism.