Incorporation of 3H-thymidine into renal cortical tissue has been studied by light microscopic autoradiography in streptozotocin-diabetic rats, uninephrectomized rats, uninephrectomized diabetic rats, insulin-treated diabetic rats and control rats. The percentage of labelled cortical nuclei (the labelling index) was determined separately in glomeruli, proximal tubules and distal tubules after 2, 4 and 6 days on autoradiographs from 1 micron thick plastic embedded sections. The incorporation of thymidine in glomerular nuclei was consistently low (less than 1%) and no differences were found between the control and experimental groups. In both proximal and distal tubules an increase in thymidine incorporation was seen on day 2 followed by a decline on days 4 and 6. The maximal labelling on day 2 in proximal tubules was 9.1% in the uninephrectomized diabetic group, 3.7% in the diabetic group and 1.4% in the uninephrectomized group. In distal tubules the corresponding values were 5.2, 3.5 and 1.1%. The increase in kidney weight after 6 days was 83, 62 and 37%, respectively. Estimates of the net increase in the number of cortical tubular cells in the different experimental groups showed that the kidney enlargement followed different patterns with respect to the extent of cellular hyperplasia and hypertrophy. The kidney growth in uninephrectomized diabetic rats was dominated by tubular cellular hyperplasia, in the diabetic group hyperplasia and hypertrophy participated to approximately the same extent, whereas cellular hypertrophy was most pronounced in the uninephrectomized animals. Nuclear labelling in the insulin-treated diabetic rats was not different from that of control rats and consequently a hyperplastic effect of streptozotocin can be ruled out.(ABSTRACT TRUNCATED AT 250 WORDS)