Certain derivatives of the 8-aminoquinolines have been shown to affect some blood constituents and haemopoiesis, to induce functional changes in the central nervous system, and to cause other organ lesions. The 8-aminoquinolines vary widely in their toxicity and ability to induce tissue damage in different laboratory animals. In the present study, the subacute toxicity of primaquine was studied in beagle dogs, rhesus monkeys, and albino rats. Based on body weight, the dog was more sensitive to primaquine than the monkey, while the rat was the least sensitive. In all three species, primaquine elevated the levels of serum transaminases, decreased the level of fasting blood glucose, and caused inflammatory and degenerative changes in the liver and kidneys. In addition, primaquine caused pneumonia and elevation of serum haptoglobin in the dog; erythrocytopenia in the monkey; reticulocytosis and the presence of nucleated erythrocytes in the rat; methaemoglobinaemia, thrombocytopenia, and inflammatory and degenerative changes of striated muscle (including the myocardium, diaphragm, tongue, and skeletal muscle) in the dog and rat; oedema and gliosis of the cerebral cortex in some monkeys; lymphoid depletion in the dog and monkey; and bile duct hyperplasia in some rats.