Regional infarction of the left ventricle is followed by hypertrophy of the viable myocardium. This compensatory growth of cardiac myocytes requires induction of gene transcription and synthesis of proteins. In this study, we examined the expression of the immediate-early response gene c-fos following ligation of the left coronary artery in rat hearts. RNase protection assay demonstrated a rapid increase in the c-fos mRNA level in the ventricular myocardium. After two days of infarction, the c-fos expression was attenuated and was comparable to that observed in sham-operated control hearts. In situ tissue distribution of Fos protein-like immunoreactivity revealed the appearance of positively stained cells adjacent to the lateral border of the ischaemic myocardium, in the left ventricular subendocardial areas, in the papillary muscles of the left ventricle, in the proximity of great transmural vessels, and focally in the normo-perfused subepicardial myocardium. Double staining using antibodies recognizing the Fos protein and alpha-actinin, confirmed that the accumulation of nuclear Fos protein-like immunoreactivity was mainly seen in the cardiac myocytes. However, double staining of the Fos protein and Hoechst DNA labelling showed that detectable immunoreactivity occurred only in a limited proportion of the total nuclei present in these myocardial regions. Moreover, the regions showing c-fos activation correspond to the areas in which the appearance of subsequent growth responses are most pronounced following myocardial infarction. The present results therefore indicate that an early and regional c-fos activation is taking place in viable cardiac myocytes following left coronary artery ligation, and that c-fos is a possible regulating factor of sequential events leading to altered pattern of gene expression and protein synthesis in the hypertrophying heart.