The aim of this research was to assess left ventricular myosin alterations of Sh, rats during the various phases in which primitive hypertension developed. Two groups of animals were examined: one comprised spontaneously hypertension rats (Shr) and the other controls. Both groups were studied from the 5th to 31st week of life. The Shrs became hypertensive at 9 weeks of age. The animals were sacrificed at: 5-7-9-12-16-24-31 weeks of age. The left ventricles were separated from their hearts and native myosin in non dissociated conditions was then obtained by polyacrylamide gel electrophoresis; heavy chains and light chains were separated with S.D.S. One of the findings showed that in the control rats the myosin iso-enzyme V1 always prevailed over the myosin iso-enzyme V3 during all the stages of life examined. In the Shrs, starting from the iso-enzyme V3 increased proportionally. The heavy chains showed the same behavior as the native protein while there were no significant differences for the light chains in both groups of animals. In conclusion, our findings show that the structural and biochemical compensation variations which several authors have demonstrated in other conditions of experimental hypertension occur in Sh rats as well.