The signaling cascades evoked by G protein-coupled receptors are a predominant mechanism of cellular communication. The regulators of G protein signaling (RGS) comprise a family of proteins that attenuate G protein-mediated signal transduction. Here we report the characterization of RGS13, the smallest member of the RGS family, which has been cloned from human lung. RGS13 has been found most abundantly in human tonsil, followed by thymus, lung, lymph node, and spleen. RGS13 is a GTPase-activating protein for Galpha(i) and Galpha(o) but not Galpha(s). RGS13 binds Galpha(q) in the presence of aluminum magnesium fluoride, suggesting that it bears GTPase-activating protein activity toward Galpha(q). RGS13 blocks MAPK activity induced by Galpha(i)- or Galpha(q)-coupled receptors. RGS13 also attenuates GTPase-deficient Galpha(q) (Galpha(q)QL) mediated cAMP response element activation but not transcription evoked by constitutively active Galpha(12) or Galpha(13). Surprisingly, RGS13 inhibits cAMP generation elicited by stimulation of the beta(2)-adrenergic receptor. These data suggest that RGS13 may regulate Galpha(i)-, Galpha(q)-, and Galpha(s)-coupled signaling cascades.