GPR55 is a G protein-coupled receptor that is proposed as a novel type of cannabinoid receptor. Lysophosphatidylinositol is an endogenous ligand for GPR55. The physiological roles of GPR55 have not yet been elucidated in detail. In the present study, the expression of Gpr55 mRNA was evaluated in various mouse tissues and organs using real-time RT-PCR. Gpr55 mRNA expression was highest in testis, the male reproductive system, among mouse tissues. Gpr55 mRNA expression was high in immune organs such as the spleen, lymph nodes, and thymus. Gpr55 mRNA was also detected in the small and large intestines. The expression of Gpr55 mRNA was relatively low in a mouse brain. The distribution of Gpr55 in mice is very similar to that in humans, however, the rank order was somewhat different. The sub-fractionation revealed that Gpr55 mRNA was expressed in both germinal cells and somatic cells in the testis. In the small intestine, Gpr55 was expressed in the duodenum, jejunum, and ileum. Gpr55 was highly expressed in B and T lymphocytes and dendritic cells in the mouse spleen.