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In the present research we tried to prove, that specific myosin heavy chain is relat­ed to a prevalent metabolic type and that activ­ities of succinate-dehydrogenase and a glycerophosphate dehydrogenase in the same fibre types can be different in different muscles. We defined the muscle fibre types (I, IIA and IIB) on transversal sections of frozen muscles accord­ing to their myofibrillar ATPase activity in alka­line and acidic media. We excised extensor digitorum longus, tibialis anterior and diaphragm muscles of five Wistar rats. In the same fibres we immunohistochemically demonstrated myosin heavy chains (MHC) I, IIA, IIB and IIX and histochemically and histophotometrically deter­mined the activities of succinate-dehydrogenase and glycerophosphate dehydrogenase. On the basis of the measured values we differ­entiated fibres in oxydative, glycolytic and oxydative-glycolytic. We found out, that MHC I are present in type I fibres (in extensor digitorum longus muscle also in type IIA fibres), MHC IIA and IIB are present in IIA fibres and also in IIB fibres in all muscles. We defined MHC IIX in fibres which are histochemically type I, IIA and IIB. Anyhow, we found no pure IIX fibres. The greatest percentage of oxidative fibres was found in diaphragm, of gly­colytic in extensor digitorum longus and of oxydative-glycolytic in the outer portion of tibialis anterior muscle. We conclude, that there is no complete corre­lation between myosin heavy chains and the prevalent type of metabolism.It is possible that some other MHC exist, which are connected to more glycolytic metabolic path­ways and are present in fibres together with already known MHC. The wide spectrum of metabolic properties of muscle fibre types can also result from the level of thyroid hormons and local factors, such as capillary supply, connec­tive tissue, tension and others.