A combined differential and density gradient centrifugation procedure was used to study the subcellular localisation of the mammalian tachykinins in rat caudateputamen and substantia nigra. Substance P, neurokinin A, neuropeptide K, and neurokinin B were found to be concentrated in the synaptosomal fractions and in fractions containing heavy synaptic vesicles in both regions studied. In contrast, the catecholamines dopamine and noradrenaline had a more widespread distribution throughout the gradient. HPLC analysis of the immunoreactivity recovered showed that the tachykinin immunoreactivity coeluted with the relevant synthetic tachykinins, except in the soluble gradient fraction where neurokinin A immunoreactivity eluted in position consistent with neurokinin A3-10. These results suggest that, in the basal ganglia, the mammalian tachykinins are localised in fractions containing large dense cored synaptic vesicles. This vesicular localisation would be consistent with the proposed role of the tachykinins as neurotransmitters and neuromodulators.