Co-localization of CCK(B) and D2 receptor mRNAs was analysed in adjacent sections of nucleus caudate putamen of the rat by means of in situ hybridization. A distinct subpopulation of D2 mRNA containing striatal nerve cells (in the order of 10%) was shown to co-express CCK(B) and D2 receptor mRNAs. All the CCK(B) receptor mRNA-positive nerve cells were co-localized with D2 receptor mRNA. These results give one morphological substrate to the previously demonstrated CCK(B)/D2 receptor interactions in the striatum.