Diabetic renal growth, regression and regrowth was studied using stereological methods on perfusion-fixed rat kidneys. The study lasted 13 weeks and comprised one control group and three diabetic groups. The first diabetic group was hyperglycaemic for 13 weeks. The second group was hyperglycaemic for 10 weeks and then normoglycaemic for 3 weeks. The third group was similar to the second group except that during the last week the animals were again hyperglycaemic. Using an optical dissector on the plastic-embedded kidney slices, the number and size of proximal and distal tubular cells were estimated. The number of proximal and distal tubular cells increased by 37% and 36% during 13 weeks of experimental diabetes and the mean volume of the proximal tubular cells increased by 12% whereas the 16% increase in mean tubular cell volume was only borderline significant as compared to the control group. Normoglycaemia for 3 weeks normalized the mean volume of distal tubular cells but the proximal tubular cells tended to be 7% smaller than those in control rats. The number of proximal cells remained increased by 21% compared with the control rats and the number of distal tubular cells retained a 17% insignificant increase. After regrowth the volume of proximal tubular cells was 20% greater than in the second diabetic group and the other parameters were unchanged. In conclusion, 13 weeks of experimental diabetes induced formation of 36% more tubular cells that were enlarged only by about 14%. Normoglycaemia for 3 weeks failed to normalize the cell number. Repeated hyperglycaemia for 1 week after 2 weeks of normoglycaemia increased the size of the proximal tubular cells.