1. The response to 60 min ischaemia was studied in male uninephrectomized 30-, 60-and 90-day-old rats in regard to changes of renal cytochrome oxidase activity and the effect on systemic arterial blood pressure. Mortality due to renal failure, changes in plasma urea concentration and in kidney morphology were used as indicators of the renal response.

2. Mortality was lowest in 30-day-old animals (13%) and highest in 60-day-old animals (77%). Renal cytochrome oxidase activity increased by 44% during the same period. The mortality decreased to 34% between the ages of 60 and 90 days without change in renal content of cytochrome oxidase; survival time after renal ischaemia was prolonged from 2 to 4 days.

3. Fifty days after renal ischaemia plasma urea concentration correlated positively with kidney weight. Both variables were increased in rats exposed to ischaemia at the ages of 60 and 90 days. The kidneys of these rats exhibited lesions. No relation was found between the degree of renal damage and blood pressure.

4. It is concluded that (a) the kidney of immature rats is more resistant to temporary ischaemia due to immaturity of renal oxidative metabolism, (b) the ability to survive ischaemic renal damage in less-resistant mature rats increases with age and (c) the postischaemic impairment of renal function does not influence systemic arterial blood pressure.

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