<正> It has been known that the antimalarial drugs of aminoquinolinesincluding 8-aminoquinolines may cause inhibition of heart conduction or evenAdams-Stokes syndrome

but their morphologic bases were obscure.The purposeof the present study is to define the histological and ultrastructural features ofprimaquine in mammals including dogs

monkeys and rats in the subacute toxicexperiments with the stress on the morphological alteration of the heart.In most（11/16）of the intoxicated dogs

granular degeneration

eosinophilic degeneration

coagulation necrosis

vacuolar degeneration and colliquative myocytolysis werefound in the myocardium and conduction systems.Under electron microscopicstudy

the myocardial cells of intoxicated dogs showed swelling of mitochondria

dilatation of transverse tubules and sarcoplasmic reticulum

fusion or dissolutionof myofibrils

deformity of nuclei

clumping of chromatin

and eventually cellnecrosis.The severities of lesions were dose dependent.These lesions weresupposed to be the morphologic evidence of the toxic symptoms and signs caused byprimaquine.A few rats also revealed myocardial eosinophilic degeneration.Noimportant cardiac lesions were found in the monkeys

but the pathologic changesin the liver were obvious.Different species of laboratory animals showed differentorgan sensitivities to primaquine

but the heart lesions should be considered as animportant toxic effect of primaquine.