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Twenty-one Shipibo Indian men were compared to 21 Mestizo men in a study of heat tolerance in the Amazon jungle of Peru, both groups walked together in the afternoon sun for 12 days. The first six were at a slow rate (5 km/hr); the second six at a faster rate (8.5 km/hr). At the 5 km/hr rate the Shipibo sweated more and showed less cardiovascular stress than the Mestizos. At the 8.5 km/hr walking rate the Shipibo still sweated more, but did not differ in other measures. Since both groups seemed well acclimatized at the outset, the Shipibo may have a propensity to be less stressed at lower work loads in the heat.