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The G-protein coupled cannabinoid receptors CB1 and CB2 are activated by Δ9-tetrahydrocannabinol, the psychoactive ingredient of cannabis, and mediate physiological effects of endogenous cannabinoids (‘endocannabinoids’). CB1 genes have been identified in mammals, birds, amphibians and fish, whilst CB2 genes have been identified in mammals and in the puffer fish Fugu rubripes. Therefore, both CB1 and CB2 receptors probably occur throughout the vertebrates. However, cannabinoid receptor genes have yet to be identified in any invertebrate species and the evolutionary origin of cannabinoid receptors is unknown. Here we report the identification of CiCBR, a G-protein coupled receptor in a deuterostomian invertebrate – the urochordate Ciona intestinalis – that is orthologous to vertebrate cannabinoid receptors. The CiCBR cDNA encodes a protein with a predicted length (423 amino-acids) that is the intermediate of human CB1 (472 amino-acids) and human CB2 (360-amino-acid) receptors. Interestingly, the protein-coding region of the CiCBR gene is interrupted by seven introns, unlike in vertebrate cannabinoid receptor genes where the protein-coding region is typically intronless. Phylogenetic analysis revealed that CiCBR forms a clade with vertebrate cannabinoid receptors but is positioned outside the CB1 and CB2 clades of a phylogenetic tree, indicating that the common ancestor of CiCBR and vertebrate cannabinoid receptors predates a gene (genome) duplication event that gave rise to CB1- and CB2-type receptors in vertebrates. Importantly, the discovery of CiCBR and the absence of orthologues of CiCBR in protostomian invertebrates such as Drosophila melanogaster and Caenorhabditis elegans indicate that the ancestor of vertebrate CB1 and CB2 cannabinoid receptors originated in a deuterostomian invertebrate. [Copyright &y& Elsevier]