We have developed a cell model for loss of differentiated gene expression in cellular aging. Over long periods in culture, bovine adrenocortical cells lose expression of a specialized gene, steroid 17 alpha-hydroxylase. The decline in expression of 17 alpha-hydroxylase in mass cultures and clones of bovine adrenocortical cells is the result of a phenotypic switching process which yields a mixture of cells that can express 17 alpha-hydroxylase after induction with cyclic AMP and cells that are incapable of expression of 17 alpha-hydroxylase. In the experimental portion of the work, bovine adrenocortical cells were grown in culture in colonies, stimulated with cyclic AMP to induce 17 alpha-hydroxylase, then fixed and hybridized in situ with labeled 17 alpha-hydroxylase cDNA. Separate images of Giemsa-stained cell colonies and of their hybridization patterns were digitized and combined as stylized representations of the colonies for comparison with those produced by a computer simulation. A program for the simulation of growth of colonies of bovine adrenocortical cells in senescence with phenotypic switching of 17 alpha-hydroxylase is presented. Comparison between simulated colonies and real colonies shows that the model accurately simulates colony growth and phenotypic switching. It suggests that the probability of phenotypic switching per cell generation is in the range of 0.03 to 0.06. The major variable among colonies is division probability, consistent with observations in this and other cell culture systems that clones differ widely in replicative potential. Thus, phenotypic switching of 17 alpha-hydroxylase in adrenocortical cells may be modelled using simple assumptions.