Rats were anesthetized with fluothane and implanted in the caudate nucleus with a push-pull cannula supplied with artificial CSF containing the tritiated precursor of dopamine (DA), [3H]tyrosine. Total DA and dihydroxyphenylacetic acid (DOPAC) were measured in successive 20 min fractions using high performance liquid chromatography and electrochemical detection. Radioisotopic counting of the peaks permitted the calculation of the specific activity of both DA and DOPAC released into the extracellular space. Local applications of potassium (8, 16 and 32 mM) induced a dose-dependent increase of the release of DA with a decrease of its specific activity as evidence of the involvement of a stored DA pool. Base release of DOPAC was increased by repeating potassium applications with a temporary decrease during the applications. Superfusion with alpha-methyl-p-tyrosine produced a decrease of both the [3H]DA and total DA with a simultaneous decrease of its specific activity. This decrease was considered to be an indicator of the involvement by synthesis inhibition of the stored amine, but the simultaneous decrease of the specific activity of DOPAC suggests that this release was intraterminal. These results constitute the first direct observation that DA terminals act with two separate pools (stored and releasable) and suggest that the stored amine is preferentially released intraterminally. Systemic injection of reserpine induced a decrease of the release of DA and DOPAC without alteration of DA-specific activity when the specific activity of DOPAC was lowered. From these results it is concluded that the releasable compartment of the amine is located, in part, in vesicles different in nature from the vesicles containing the stored amine.(ABSTRACT TRUNCATED AT 250 WORDS)