Problems associated with the transformation of differentiated cells in vertebrate organisms are discussed based on electron microscopical results of intermediate cells (i.e. cells with morphological characteristics of exocrine acinar cells and endocrine cells of Langerhans' islets) in the pancreas of human adults with chronic insulin-dependent diabetes mellitus. In this context, reference is made to experimental results of Scarpelli, Rao, and coworkers relating to the occurrence of hepatocyte-like cells in the pancreas of Syrian golden hamsters (Rao and Scarpelli 1980; Scarpelli and Rao 1981; Rao et al. 1983). These observations show that exocrine acinar cells of the pancreas may, even beyond the neonatal period, become transformed, depending upon different triggering stimuli, into different endocrine islet cells, or into hepatocytes, this being accomplished either directly or by new formation of cells (regeneration) with abnormal differentiation (metaplasia). Obviously, transformation is effected through a change in the activation of gene loci: the normally stably blocked genes are partially or completely deblocked for the functions of different endocrine islets cells or hepatocytes, and the original genetic expression of exocrine pancreatic functions is blocked either partially or completely. The results presented and quoted in this paper suggest that in all differentiated cells derived from the endoderm of the foregut, such as duct cells, exocrine and endocrine pancreatic cells, and hepatocytes, functional programs are retained which can be modified in the manner quoted to enable partial or complete transformation into one or another of these differentiated cells in the adult organism.