Sheep latissimus dorsi muscle was electrically trained, thereby inducing fast-to-slow fibre-type transformation. Using a combination of one- and two-dimensional gel electrophoresis techniques with computer analysis, we have analysed altered expression of contractile protein isoforms at protein and mRNA level over a time course of electrical training extending to 5 months. Myosin heavy chain and regulatory myosin light chain analysis showed predominant expression of their slow isoforms (86% and 92%, respectively) after 3 months of training. At the same time point, however, tropomyosin analysis revealed that the slow isoform of the alpha-subunit accounted for 64% of the total alpha expression. Troponin T isoform switching proceeded more slowly over the same time course than tropomyosin and the thick filament proteins studied. Troponin T analysis revealed 5 fast and 2 slow isoforms in the sheep, of which the second slow isoform only became clearly visible after 5 months' training. At this time point the two slow isoforms were more predominant than their fast counterparts. This suggests that a wide heterogeneity of fast and slow isoform combinations are possible in the thin filament of skeletal muscle.