Heat exhaustion (collapse with rectal temperature of, or higher than 38 degrees C) is the most common major medical complication of fun runs and is caused by dehydration and impaired heat loss with, or without, hypoglycaemia. All patients with heat exhaustion after the City-to-Surf runs from 1977 to 1979 were managed in a medical centre established at the finish of the course. Several methods of management of heat exhaustion are evaluated. Patients were allocated retrospectively to four primary treatment groups: (i) treated with ice-wet towels only; (ii) treated with ice-wet towels and intravenously administered fluids; (iii) treated with intravenously administered fluids and ice-cold packs applied to the neck, axillae, and groins; and (iv) treated with intravenously administered fluids only. There was no death or prolonged morbidity in any treatment group. In Group 1 (n=11), the mean initial rectal temperature was 40.2 +/- 1.5 degrees C. There were insufficient data to assess the mean time taken for the temperature to fall to 38 degrees C. In Group 2(n=16), the mean initial rectal temperature was higher than 40.9 +/- 1.1 degrees C. In three patients, the temperature failed to fall to 38 degrees C within 90 minutes. For the remainder, the mean time taken for the temperature to fall to 38 degrees C was 30 minutes. In Group 3 (n=16), the mean initial rectum temperature was higher than 41.2 +/- 1.0 degrees C. One patient, who had been febrile immediately before the run, was discharged with a temperature of 38.8 degrees C. For the remainder, the mean time taken for the temperature to fall to 38 degrees C was 36 +/- 10 minutes. In Group 4 (n=13), the mean initial rectal temperature was 39.6 +/- 1.1 degrees C and the mean time taken for the temperature to fall to 38 degrees C was 21 +/- 16 minutes. Heat exhaustion in fun-run casualties may be safely and effectively treated by rapid intravenous infusion of fluids with, or without, application of cold packs to the neck, axillae, and groins. The application of ice-wet towels is contraindicated.