The acute oesophageal response, after localized mediastinal irradiation in unanaesthetized mice, was assessed with a sequential, non-invasive and clinically orientated assay. Changes in body weight, after single and fractionated X-ray doses (8 fractions in 8 or 15 days and 4 fractions in 4 or 13 days), were recorded and the nadir of weight loss for each regime was used as the end-point. In our studies, quantal and non-quantal analyses of the weight loss data gave good dose-response relationships. The shapes of the dose-effect curves for single doses, 4F/4 days and 8F/8 days showed a remarkable degree of radioresistance, which became less apparent when the overall time was increased to 13 and 15 days for 4 and 8 fractions, respectively. The alpha/beta values (7-17 Gy) are within the range found for other acutely responding normal tissues. Histopathological changes were also assessed and the structural changes observed in the oesophageal epithelium, after a single dose of 34 Gy or after 46 Gy given as 3F/6 days, correlated with the gross changes observed in body weight.