Go.alpha. is a 39-kDa guanine nucleotide-binding protein (G protein) similar in structure and function to Gs.alpha. and Gi.alpha. of the adenylate cyclase complex and to transducin (Gt.alpha.) of the retinal photon receptor system. Although expression of Go.alpha. protein has been reported to be tissue-specific, other workers have found Go.alpha. mRNA in all rat tissues examined. In order to clarify this contradiction, studies to verify the distribution of Go.alpha. mRNA and protein in bovine and rat tissues were performed. Tissues were screened for the presence of Go.alpha. mRNA by use of a series of restriction fragments of a bovine retinal cDNA clone, .lambda.GO9, and oligonucleotide probes complementary to sequences specific among G.alpha. subunits for the 5'' untranslated and coding regions of Go.alpha.. These probes hybridized predominantly with mRNA of 4.0 and 3.0 kb in bovine brain and retina. A 2.0-kb mRNA in retina also hybridized strongly with the cDNA but weakly with the oligonucleotide probes. In bovine lung, two mRNAs of 1.6 and 1.8 kb hybridized with the cDNA while only the 1.6-kb species hybridized with the coding-region oligonucleotide. In bovine heart, only a 4.0-kb mRNA was detected and in amounts much less than those in the other tissues. A similar distribution of Go.alpha. mRNAs was seen in rat tissues. In bovine tissues, Go.alpha. protein was identified with rabbit polyclonal antibodies directed against purified bovine brain Go.alpha.. An immunoreactive 39-kDa membrane protein was found principally in retina brain, and in a lesser amount in heart. Thus, in the rat and bovine tissues examined, Go.alpha. or Go.alpha.-like mRNAs, as well as Go.alpha. protein, are expressed in a tissue-specific manner.