The effect of the d- and l-isomers of amphetamine on the activity of dopaminergic neurons in the substantia nigra zona compacta and noradrenergic neurons in the locus coeruleus was studied in anesthetized and gallamine paralyzed rats using single unit recording techniques. d- and l-Amphetamine administered intravenously were equally effective in depressing the activity of locus coeruleus cells. However, although d-amphetamine was a potent inhibitor of substantia nigra dopamine containing cells, l-amphetamine was ineffective in causing more than a 45% inhibition of over half of the DA cells studied, even when given in nearly lethal doses. In the remaining dopamine neurons l-amphetamine was only 0.2 to 0.05 times as potent as d-amphetamine in producing both 50 and 100% inhibition of firing rate. These findings, when combined with recent biochemical studies by other authors, suggest that l-amphetamine, at low doses, has a preferential effect on noradrenergic as compared to dopaminergic neurons. The consequences of these findings for the interpretation of studies in which d- and l-amphetamine have been used as pharmacological tools to determine the catecholamine system responsible for a particular behavior in man and animals is discussed.