The use of multiple fractions per day (MFD) in radiotherapy requires information about the rate of repair of radiation injury. It is important to know the minimum interval between fractions necessary for maximum sparing of normal tissue damage, whether rate of repair is dependent on the size of dose per fraction and if it is different in early and late responding tissues and in tumours. To address these questions, the rate of repair between radiation dose fractions was measured in mouse skin (acute damage), mouse kidney (late damage) and a mouse tumour (carcinoma NT). Skin and kidney measurements were made using multiple split doses of X-rays, followed by a neutron top-up. For skin, faster recovery was obtained with 4.4 Gy fractions (t1/2 = 1.29 +/- 0.35 h, 95% CL) than with 10.5 Gy fractions (t1/2 = 3.46 +/- 0.88 h). In contrast, kidney showed slower recovery at a low dose per fraction of 2 Gy (t1/2 = 1.69 +/- 0.39 h) than at a higher dose of 7 Gy per fraction (t1/2 = 0.92 +/- 0.1 h). These data show that repair rate is dependent on the size of dose per fraction, but not in a simple way. T1/2 values now available for many different tissues generally lie in the range of 1-2 h, and are not correlated with proliferation status or early versus late response to treatment. At the doses used currently in clinical MFD treatments, these data indicate that damage in almost all normal tissues would increase if interfraction intervals less than 6 h were used.(ABSTRACT TRUNCATED AT 250 WORDS)