Adducin (ADD) is a heterodimeric protein involved in cellular signal transduction. A mutation in the alpha subunit affects ion transport and blood pressure in primary hypertension of Milan rats (MHS) and humans. In rats this effect is modulated by another mutation in the beta subunit. The recently described gamma subunit is a new member of the ADD family that should take the place of beta subunit in cells and tissues expressing alpha but not beta-Add. A missense mutation (Q572K) has been found in the gamma subunit of the Milan rats. Nineteen normotensive and five hypertensive inbred rat strains were genotyped for the polymorphisms in alpha, beta and gamma-Add genes. A disequilibrium was evident in the distribution of MHS-like Add genotype, being more frequent between the hypertensive than the normotensive strains (Chi-Square = 13.03, p = 0.0003). In kidney, brain, spleen, liver and heart a cDNA differing from gamma subunit by an in-frame insertion of 96 nucleotides, was found by PCR amplification and confirmed by RNase protection analysis. The rat gamma-Add gene was localized to chromosome 1q55 by fluorescence in situ hybridization.