Cysteine string proteins (csps) are a recently discovered class of cysteine-rich proteins. They have been shown to associate preferentially with synaptic vesicle fractions of Torpedo electric organ or rat brain where they have been implicated in events associated with transmitter secretion. However, to date there has been no information concerning the distribution of csps in rat tissues. We investigated the localization of csps in the rat retina and CNS using immunohistochemistry with affinity purified anti-csp antibodies. Specific csp immunoreactivity having a punctate appearance is present throughout the neuraxis. Csp immunoreactivity is particularly abundant in synapse-rich regions including those of the retina, main olfactory bulb, hippocampal formation, and cerebellum. White matter tracts are devoid of csp immunoreactivity. Neuromuscular junctions show strong csp immunoreactivity. This localization of csp immunoreactivity is compatible with a role for csps in presynaptic events at a wide variety of synapses. Immunohistochemical analysis of a non-neuronal, secretory tissue, the adrenal gland, reveals prominent csp immunoreactivity in the chromaffin cells of the adrenal medulla. However, csp immunoreactivity is not detected in adrenal cortical regions. These findings are confirmed and extended by immunoblot and Northern analyses which identify a 35 kDa and a 5 kb product, respectively, in extracts of adrenal. The presence of csps in the adrenal medulla suggests that these proteins may also participate in secretion-related events in certain non-neuronal cells.