We previously demonstrated (Chakrabarti, et al., 2001) that in vivo phosphorylation of the Gbeta subunit of G proteins, via protein kinase A (PKA) and protein kinase C (PKC), is dramatically increased following chronic morphine. The present study investigates the PKC isoform selectivity of Gbeta phosphorylation and the consequences thereof on the ability of Gbetagamma to stimulate adenylyl cyclase II (ACII). The catalytic subunit of PKC and PKA, as well as the conventional PKC isoform PKCgamma, was effective in phosphorylating Gbeta. In contrast, Gbeta was only minimally phosphorylated by another conventional isoform, PKCalpha or the atypical isoform PKCzeta. In the presence of activated recombinant Gsalpha, ACII activity was dose dependently stimulated by G(betagamma), the magnitude of which was dependent upon its phosphorylation state. The increment in ACII activity produced by Gbetagamma was increased approximately 2-fold following in vitro phosphorylation by the catalytic subunit of either PKA or PKC. In contrast, the concomitant or sequential phosphorylation of Gbetagamma by PKA and PKC catalytic subunits did not result in an additive enhancement of its ability to stimulate ACII and, in fact, negated the observed enhancing effect of each kinase, individually. Threonine phosphorylated G(beta) occurs naturally in the spinal cord, the levels of which are augmented (approximately 60%) by chronic morphine. The natural occurrence of phosphorylated Gbeta in spinal cord, its up-regulation following chronic morphine and the augmented ability of phosphorylated Gbetagamma to stimulate ACII activity, in the aggregate, indicate that phosphorylation of Gbeta could be a regulatory mechanism causally associated with altered cellular signaling.