1. Di(beta-phenylisopropyl)amine (DPIA) given i.p. to mice and rats in sublethal doses caused increased motility, mild stereotypic behavior and suppression of food intake. Repeated daily doses led to enhanced motor stimulation, and in one group, 40% lethality, indicating development of "reverse tolerance". Brain monoamine modifiers prevented DPIA-induced motor activity. 2. Treatment with toxic i.p. doses of DPIA enabled determination of the LD50, which was 106.8 mg/kg for isolated mice and 89.7 mg/kg for mice kept in aggregation after dosing. Possible antidotal agents given before a high DPIA dose (LD50) protected significantly against lethality. 3. Combinations of DPIA with (+)-amphetamine in mice at lethal doses showed a subadditive synergism. 4. Effects of DPIA on the cardiovascular system, both i.v. in anesthetized rats and in isolated atrial preparations, were mainly opposite to those of (+)-amphetamine, namely decreases in blood pressure, force of contraction and heart rate.