The pharmacokinetics, physiologic effects, and the metabolization of chloroquine and hydroxychloroquine are all similar. Their concentrations in plasma and tissue are directly related to daily dosing. The highest concentrations are found in melanin-containing tissues, particularly the choroid and ciliary body of the eye. The pharmacologic effects of 4-aminoquinoline compounds are reviewed in detail. It is likely that the rheumatologic effectiveness of these agents is primarily related to lysosomal actions. The drug-induced lysosomal abnormalities include diminished vesicle fusion, diminished exocytosis, and reversible "lysosomal storage disease." It is likely that the retinal toxicity of these drugs is one manifestation of the altered lysosomal physiology involving the retinal pigmented epithelium. Tissue of retinal pigmented epithelium is similar to that of the bone-marrow-derived macrophage. Depression of extra-oculogram is an early sign of excessive dosage and can be used to measure potential toxicity during therapy with 4-aminoquinolines. Dosages ranging from 3.5 to 4.0 mg/kg per day for chloroquine and 6.0 to 6.5 mg/kg per day for hydroxychloroquine are clinically safe.