The magnitude and duration of spontaneous and of potassium-stimulated dopamine release were electrochemically measured in striatum and nucleus accumbens of chloral hydrate-anesthetized rats following [D-Pen2-D-Pen5]enkephalin, a delta opioid receptor agonist, or [Tyr-D-Ala-MePhe-Gly-ol], a mu opioid receptor agonist, microinjected directly into the voltammetric recording sites. The data show that delta receptor activation potentiated potassium-stimulated dopamine efflux in striatum and in nucleus accumbens but had no effect on spontaneous dopamine release in either region, whereas mu receptor activation produced unreliable effects in both regions, either having no effect or inhibiting stimulated dopamine efflux without affecting basal levels of extracellular dopamine in either region. The data suggest that some delta opioid receptors in the caudate-putamen and in the nucleus accumbens presynaptically enhance impulse-dependent dopamine release from nigrostriatal and mesolimbic dopamine terminals.