Activating mutations of G proteins, which are membrane signal transducers, have been associated recently with the development of various endocrine neoplasms. Mutations of two highly conserved codons, Arg201 and Gln227, in the alpha-subunit of the Gs protein, the adenylyl cyclase-stimulating protein, were first described in growth hormone-producing pituitary tumours. They resulted in constitutive activation of the alpha s-subunit by decreasing intrinsic GTPase activity. A similar mutation, affecting codon Arg179 (exon 5) in the alpha-subunit of the Gi2 protein, the adenylyl cyclase-inhibiting protein, has been described by a single group in ovarian and adrenocortical tumours. We evaluated the frequency of activating mutations in the alpha-subunit of the Gi2 protein in 18 human adrenocortical tumours. We screened exons 5 (codon Arg179) and 6 (codon gln205) for mutations by denaturing gradient gel electrophoresis analysis of leucocyte and tumoural DNA. No abnormal migration pattern was found in either exon. The absence of mutation in exon 5, which includes the Arg179 codon, was confirmed in all tumoural DNA by direct sequencing. In conclusion, we did not find any oncogenic mutations in the GTPase domain of the alpha-subunit of the Gi2 protein in adrenocortical tumours. Thus, the previously oncogenic gip2 mutations do not appear to be determinant for adrenocortical tumourigenesis.