Subtotal nephrectomy in rats, the so-called ablation model, leads to progressive destruction of the kidney and the development of chronic renal failure. We postulated that the progressive glomerular lesions were due to an early increase in the turnover of glomerular resident cells. This abnormal glomerular growth results in an increase in the glomerular volume which is detectable using morphometric measurements. We have performed autoradiographic studies, using (3)H-thymidine, and found that within two days following subtotal nephrectomy there was an increase in the glomerular mitotic index, as well as an increase in the turnover of the cells forming the arterial wall. These findings suggest that a dysregulation of cell growth is an early event in the development of glomerulosclerosis following reduction in renal mass.