The enzyme carboxypeptidase H was detected by immunohistochemistry in the striatum of adult cats and monkeys. Specific labelling was observed in the neuropil as well as in both medium-sized and large neuronal cell bodies. The distribution of neurons and neuropil expressing immunoreactivity to carboxypeptidase H was examined in relation to the pattern of immunoreactivity to the neuropeptides enkephalin and substance P. Carboxypeptidase H-like immunoreactivity was found both in zones rich and poor in immunostaining for the two peptides, but was usually denser in those striatal areas in which substance P-positive cell bodies are clustered (striosomes). The results further suggest a role for carboxypeptidase H in the metabolism of multiple neuropeptides in vivo.