Electrophoretic analyses of muscle proteins in whole muscle homogenates and single muscle fiber segments were used to examine myosin heavy chain (MHC) and myosin light chain 2 (MLC2) isoform composition and fiber type populations in soleus muscles from spontaneously hypertensive rats (SHRs) and their age-matched normotensive controls [Wistar-Kyoto (WKY) rats], at three stages in the development of high blood pressure (4 wk, 16 wk, and 24 wk of age). Demembranated (chemically skinned with 2% Triton X-100), single fiber preparations were used to determine the maximum Ca2+-activated force per cross-sectional area, calcium sensitivity, and degree of cooperativity of the contractile apparatus and Ca2+-regulatory system with respect to Ca2+. The results show that, at all ages examined, 1) SHR soleus contained a lower proportion of MHCI and MLC2 slow (MLC2s) and a higher proportion of MHCIIa, MHCIId/x, and MLC2 fast (MLC2f ) isoforms than the age-matched controls; 2) random dissection of single fibers from SHR and WKY soleus produced four populations of fibers: type I (expressing MHCI), type IIA (expressing MHCIIa), hybrid type I+IIA (coexpressing MHCI and MHCIIa), and hybrid type IIA+IID (coexpressing MHCIIa and MHCIId/x); and 3) single fiber dissection from SHR soleus yielded a lower proportion of type I fibers, a higher proportion of fast-twitch fibers (types IIA and IIA+IID), and a higher proportion of hybrid fibers (types I+IIA and IIA+IID) than the homologous muscles from the age-matched WKY rats. Because the presence of hybrid fibers is viewed as a marker of muscle transformation, these data suggest that SHR soleus undergoes transformation well into adulthood. Our data show also that, for a given fiber type, there are no significant differences between SHR and WKY soleus muscles with respect to any of the Ca2+-activation properties examined. This finding indicates that the lower specific tensions reported in the literature for SHR soleus muscles are not due to strain- or hypertension-related differences in the function of the contractile apparatus or regulatory system.