The guanosine triphosphate-binding proteins (G proteins) found in a variety of tissues transduce signals generated by ligand binding to cell surface receptors into changes in intracellular metabolism. Amino acid sequences of peptides prepared by partial proteolysis of the alpha subunit of a bovine brain G protein and the alpha subunit of rod outer-segment transducin were determined. The two proteins show regions of sequence identity as well as regions of diversity. A portion of the amino-terminal peptide sequence of each protein is highly homologous with the corresponding region in the ras protein (a protooncogene product). These similarities suggest that G proteins and ras proteins may have analogous functions.