Mouse kidneys were bilaterally irradiated with X-ray doses of either 6 or 10 Gy (equivalent to approximately 30 and 70 per cent of full tolerance respectively). After an interval of 2 or 26 weeks the mice were retreated with a range of test doses given as single or fractionated irradiation schedules. Functional kidney damage was measured (using clearance of [51Cr]EDTA) before retreatment and at monthly intervals up to 1 year after retreatment. Reirradiation tolerance was assessed from dose-response curves for renal damage in retreated mice compared with that in age matched controls which received only the second treatment. Damage from the initial radiation doses progressed, leading to a decreased reirradiation tolerance with time. There was no evidence for any recovery from functional damage in the interval between 2 and 26 weeks. These studies would strongly suggest that reirradiation of a previously irradiated kidney (even after low initial doses below tolerance) is likely to lead to severe renal damage. Despite the poor reirradiation tolerance at 26 weeks, there was no reduction in the capacity for repair of sublethal injury when fractionated irradiation was given to previously irradiated mice compared with controls.