A morphometric study was performed on moderately hyperglycaemic streptozotocin diabetic rats after 10 and 50 days of diabetes, and on groups of rats that, after initial hyperglycaemia for 50 days, were insulin treated for 2 h or for 5, 15 or 38 days. A group of hyperglycaemic diabetic animals were fasted for 18 h. Another group of rats had acute hyperglycaemia induced by intravenous glucose injection. After 10 and 50 days of diabetes, kidney weight was increased by 55 and 93%. Glomerular volume, tubule length, and tubular and interstitial volume increased in diabetic animals compared with controls. After 4 h insulin treatment, the kidney weight was 20% decreased; after 5 days it was 31% decreased. After 38 days the kidney weight was still 26% greater than in controls. In diabetic animals, 18 h fasting induced a 30% decrease in kidney weight. In normal animals, acute hyperglycaemia induced a 22% increase in kidney weight. Volume fractions of most kidney structures remained similar in all groups. However, the glomerular volume fraction was smaller during kidney enlargement, and the tubular volume fraction was larger after induced hyperglycaemia compared with controls. In conclusion, high blood glucose levels in diabetic and normal animals are associated with increased kidney weight. In hyperglycaemic diabetic animals, normalization of blood glucose after insulin treatment or fasting was followed by a decrease in kidney weight.