The effect of hyperglycemia, induced by administration of glucose, on the radiation response of the Lewis lung and EMT6 tumor models has been evaluated. Neither acute (single i.p. injection of 8 mg/glucose) nor chronic (multiple i.p. injections of 6 mg/g glucose plus glucose in the water bottles) administrations of glucose increased the radiation response of either tumor. A combination of a single i.p. injection of glucose and a reduction of the O2 content of the inspired gas to 10 per cent did by itself reduce cell survival by 55-75 per cent in the EMT6 and 80-90 per cent in the Lewis lung carcinoma. However, this treatment had little effect on the shape of the radiation dose-response curve, and simply gave rise to a parallel shift of the survival curve, indicating that this treatment had little or no specificity for hypoxic cells.