During pilgrimage season (Hajj) in Saudi Arabia 34 patients with heat stroke (HS) were centrally cannulated to assess their state of hydration and fluid requirement during cooling period. Central venous pressure (C.V.P.) measurements indicated that most victims of heat stroke had normal C.V.P. on arrival at heat stroke centres and may not be fluid depleted. Twenty-two patients (64.7%) had normal or above normal C.V.P. Twelve patients (35.3%) had zero or below zero C.V.P. Six patients (17.6%) had above 10 cmH2O (range 10-26 cmH2O) and could have developed acute congestive heat failure and pulmonary edema if they had been transfused at the standard recommended rate of 3-4 litres of fluid during an average cooling time of 1 h as has been practiced in the heat stroke centres to date. This study also showed that heat stroke patients should not be briskly transfused because the heart may be affected by heat stroke per se and an unmonitored challenge by brisk i.v. therapy during cooling (which on its own increases preload on the heart due to peripheral vasoconstriction) can lead to acute overload problems. An average of 1 litre of normal saline or Ringer's lactate (crystalloids) was sufficient to normalize C.V.P. during the cooling period and to restore an optimal state of hydration without predisposing to congestive cardiac failure and pulmonary edema--the potential to develop disastrous adult respiratory distress syndrome and disseminated intravascular coagulopathy.