Double-label in situ hybridization was used to identify the phenotypes of striatal neurons that express mRNA for cannabinoid CB(1) receptors. Simultaneous detection of multiple mRNAs was performed by combining a (35)S-labeled ribonucleotide probe for CB(1) mRNA with digoxigenin-labeled riboprobes for striatal projection neurons (preprotachykinin A, prodynorphin, and preproenkephalin mRNAs) and interneurons (vesicular acetylcholine transporter (VAChT), choline acetyltransferase (ChAT), somatostatin, and glutamic acid decarboxylase (Mr 67,000; GAD67) mRNAs). To ascertain whether CB(1) mRNA was a marker for striatal efferents, digoxigenin-labeled probes for mRNA markers of both striatonigral (prodynorphin or preprotachykinin A mRNAs), and striatopallidal (proenkephalin mRNAs) projection neurons were combined with the (35)S-labeled probe for CB(1). A mediolateral gradient in CB(1) mRNA expression was observed at rostral and mid-striatal levels; in the same coronal sections the number of silver grains per cell ranged from below the threshold of detectability at the medial and ventral poles to saturation at the dorsolateral boundary bordered by the corpus callosum. At the caudal level examined, CB(1) mRNA was denser in the ventral sector relative to the dorsal sector. Virtually all neurons expressing mRNA markers for striatal projection neurons colocalized CB(1) mRNA. Combining a (35)S-labeled riboprobe for CB(1) with digoxigenin-labeled riboprobes for both preproenkephalin and prodynorphin confirmed localization of CB(1) mRNA to striatonigral and striatopallidal neurons expressing prodynorphin and preproenkephalin mRNAs, respectively. However, CB(1) mRNA-positive cells that failed to coexpress the other markers were also apparent. CB(1) mRNA was localized to putative GABAergic interneurons that express high levels of GAD67 mRNA. These interneurons enable functional interactions between the direct and indirect striatal output pathways. By contrast, aspiny interneurons that express preprosomatostatin mRNA and cholinergic interneurons that coexpress ChAT and VAChT mRNAs were CB(1) mRNA-negative. The present data provide direct evidence that cannabinoid receptors are synthesized in striatonigral neurons that contain dynorphin and substance P and striatopallidal neurons that contain enkephalin. By contrast, local circuit neurons in striatum that contain somatostatin or acetylcholine do not synthesize cannabinoid receptors. Published 2000 Wiley-Liss, Inc.