Adducin (ADD) is a heterodimeric protein involved in cellular signal transduction. A mutation in the α 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 β subunit. The recently described γ subunit is a new member of the ADD family that should take the place of β subunit in cells and tissues expressing α but not β-Add.A missense mutation (Q 5 7 2 K) has been found in the γ subunit of the Milan rats. Nineteen normotensive and five hypertensive inbred rat strains were genotyped for the polymorphisms in α, β and γ-Addgenes. A disequilibrium was evident in the distribution of MHS-likeAddgenotype, 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 γ subunit by an in-frame insertion of 96 nucleotides, was found by PCR amplification and confirmed by RNase protection analysis. The rat γ-Addgene was localized to chromosome 1q55 by fluorescencein situhybridization.