Slow (m.soleus) and fast (m.tibialis anterior) muscles of the rabbit were subjected to indirect long-term intermittent stimulation (3 weeks, 8 hrs daily) with a frequency pattern of 10 imp/sec. Whereas no changes were observed in case of the slow muscle, stimulation induced profound changes in the fast tibialis anterior muscle. These consisted in a rearrangement of the enzyme activity pattern of energy-supplying metabolism, e.g. decrease in glycogenolytic and glycolytic enzyme activities and severalfold increase in key enzymes of aerobic endoxidation of substrates in beta-oxidation and the citric acid cycle. Concomitant with the increase in aerobic oxidative capacity, there was an increased resistance to fatigue. Histochemical studies revealed a strong increase in mitochondria of all fibres. The bimodal distribution of fibre cross-sectional area in the normal tibialis anterior muscle was changed by stimulation into a more homogeneous population of fibres with a smaller cross-sectional area. Despite a 50% increase in time to peak of isometric twitch contraction no changes were observed in the fibre population with regard to myofibrillar ATPase reaction in quantitative evaluation of whole cross-sections of the muscles. The percentage of fibres histochemically classified as slow amounted to 2.8% and 3.1% in control and stimulated tibialis anterior muscle. Nevertheless the data suggest a transformation of the fibre population under the influence of long-term intermittent stimulation.