The inheritance of variability in the behavioral and physiologic effects of d-amphetamine is being studied. A small dose of amphetamine (0.3 mgm/kg) is administered to normal volunteer twins after medical and psychiatric screening. Preliminary data suggests stable and heritable responses in: 1) behavioral ratings; 2) cortical evoked responses; 3) diurnal cycle of motor activity; 4) neuroendocrine response and; 5) pharmacokinetics of amphetamine. We do not find adrenergic receptor response to be stable and heritable. We have developed a new behavioral rating system to assist us in this work.