One hundred and eleven cases of heat effects are described, divided as follows: Heat exhaustion and collapse: Mild 52, Moderate 39, Severe 9 =100; Heat Hyperpvrexia: 11; Mortality: Nil; Morbidity: Average stay in hospital: 9.5 days; Average sick leave in the hills: 10 days; (This figure is not really accurate as it fails to take into account that several cases were due for routine hill leave and were sent direct from hospital with their unit parties.); Cases repatriated 3. Clinical details of all cases of hyperpyrexia and of the severe cases of heat exhaustion are given. Other cases have been treated statistically. Laboratory tests were subordinated to clinical findings and only essential investigations (examination for malarial parasites, etc.) were carried out. Cases were considered in two groups, those with considerable tropical experience, who had spent at least one hot season in India, and those with under a year's tropical service. A description of the Hospital and the facilities available is given, and of the advance preparations made to deal with the expected occurrence of cases as the hot season reached its climax. Details of the general lines of treatment of different types of case have been given in full. The medical literature on the subject has been discussed, from Haldane's pioneer research to the scientific analysis of Ladell and his colleagues in 1944 and Professor Maegraith's summary in this year's British Medical Journal. A suggestion is made of the possible value of pentothal sodium as an adjuvant to the treatment of hyperpyrexia.