The evolution of glomerulosclerosis consists of a progressive increase in mesangial matrix with gradual glomerular obliteration. The sclerotic process is thought to be irreversible and include a progressive loss of glomerular cells. To investigate this process, we selected mice transgenic for bovine growth hormone because they develop progressive glomerulosclerosis and renal failure. The sequence of histologic events in the growth hormone mice consists initially of an increase in the number of centrolobular glomerular cells, followed by an accumulation of extracellular matrix. This is accompanied by an increase in glomerular size which is disproportionate to the overall increment in kidney or body weight. The [3H] thymidine labeling index of the cells of the glomerular tuft was assessed before the development of recognizable sclerosis and at a time when the sclerosis was far advanced. The labeling index was more than five-fold increased over controls at the early time point. Contrary to what was expected, the labeling index remained at the same high levels in densely sclerotic glomeruli at the late time point. In conclusion, increased cell turnover is a significant component of the sclerotic process both at the onset and in the late stages of this model.