Several guanine nucleotide analogs, in one series of which a hydrogen on the 2-amino group is replaced with the p-n-butylphenyl group (BuPGNP derivatives), were used to probe the GTP binding domain of bovine transducin. The order of apparent binding affinities in a series of nucleoside 5'-triphosphates was GTP gamma S greater than GTP approximately BuPGTP greater than dGTP approximately ITP much greater than ATP, values which were 30-100 times higher than affinities of the corresponding 5'-diphosphates. A derivative bearing a 6-aminohexylamino group on the gamma-phosphate, BuPGTP X C6, had a 60-fold lower affinity compared to BuPGTP. In contrast, the p-n-butylphenyl substituent on the 2-amino group had little effect on the binding affinity relative to GTP. Substitutions at the 2-amino group had little effect on either the hydrolysis of the derivatives by the GTPase activity associated with the alpha-subunit of transducin or the activation of cGMP phosphodesterase. The results indicate that the GTP binding domain of transducin is similar in tertiary structure to the corresponding domain of EF-Tu. The 5'-phosphates of GTP are oriented in the binding site of transducin so that the bulky C6 group of BuPGTP X C6 dramatically interferes with binding. The 2-amino group on the guanine ring is probably located at the periphery of the binding site, with the p-n-butylphenyl substituent of BuPGTP facing outward and only weakly interacting with the protein. BuPGTP should be an excellent parent compound for development of novel probes of G-protein interactions with other cellular proteins involved in receptor signal transduction.