The indirect immunofluorescence technique to detect endogenous C-flanking peptide of neuropeptide Y (NPY), C-PON, in combination with NADPH-diaphorase (NADPH-d) histochemistry, were applied to the same sections to establish whether C-PON containing cell bodies in the rat striatum can be labelled by their content of NADPH-diaphorase activity. NADPH-diaphorase activity proved to be a reliable marker for these positive C-PON neurons. Our results suggest that, in the rat striatum, C-PON, SOM and NPY co-exist in the same cells.