By examining the blood pressure in dogs and electrocardiogram in rabbits treated with drin compounds, the author obtained the following results: 1. Drin compounds, when injected intravenously in dogs, cause a transitory lowering of blood pressure. But when generalized convulsions occur, blood pressure rises and becomes markedly labile. 2. Administration of atropine does not inhibit the effect of drin compounds on blood pressure. 3. Simultaneous administration of dieldrin and adrenaline inhibits blood pressure lowering effect of dieldrin. 4. Ganglion blocking agents such as chlorpromazine and tetraethylammonium do not inhibit the effect of dieldrin on bood pressure. 5. Administration of drin compounds slows the heart rate in rabbits. 6. In rabbits given drin compounds, electrocardiograms often demonstrate depressed ST segments and low to inverted T waves. 7. In rabbits given a large amount of drin compounds, electrocardiograms may show left bundle branch block, atrial fibrillation, ventricular premature beat and multifocal idioventricular rhythm. 8. In rabbits treated with a small dosage of drin compounds for a prolonged period, electrocardiograms show no significant changes.