Intracellular localization of FBPase in the cardiac muscle of the pig was studied by immunohistochemistry. In contrast to data from skeletal muscle [Gizak, A., Rakus, D. and Dzugaj, A. (2003) Histol. Histopathol. 18, 135-142], in cardiomyocytes FBPase was present not only in the cytoplasm, but surprisingly, also in the nucleus. Results of the microscopic investigation were confirmed by immunoblotting, measurement of FBPase activity in isolated cardiomyocyte nuclei and by determination of the nuclear FBPase I(0.5) toward adenosine monophosphate (AMP), which was the same as for the purified enzyme.