We recently demonstrated that two exclusively Gi-coupled isoforms of the mouse EP3 receptor, EP3alpha and beta, with different carboxyl-terminal tails, differed in agonist-independent constitutive Gi activity, and the carboxyl-terminal tail-truncated receptor showed full constitutive activity (Hasegawa, H., Negishi, M., and Ichikawa, A. (1996) J. Biol. Chem. 271, 1857-1860). Here we further examined Gi and Gs activities of the third isoform, EP3gamma, coupled to both Gi and Gs. The My receptor showed mostly full constitutive Gi activity and agonist-dependent Gs activity. The truncated receptor also showed agonist-dependent Gs activity, but the level was lower than that of the EP3gamma receptor. Thus, the carboxyl-terminal tail would differentially regulate Gi and Gs activities of the EP3 receptor.