Heat acclimation (HA) and forced water intake (FWI) have both been found to improve the endurance of human subjects working in hot environments. Therefore, we studied the interaction between HA and FWI. Prior to any treatment (control, AI and BI) the subjects (n = 9) underwent a heat tolerance (HT) test. Thereafter, they were divided into two groups. The first (n = 5) were heat-acclimated (AII), underwent a second HT test, doubled their normal daily water intake for 1 week (AIII), and underwent a third HT test; the second group (n = 4) were subjected to the same protocol, except that the FWI came before and during HA (BII). It was found that both regimens (phases AII and BII) significantly increased work duration. Although the results of the two methods were similar, their combination somewhat lengthened work tolerance time (phases AIII, BIII). Maximal oxygen uptake did not change after HA (BII) or FWI (AII), but the maximal values were attained at significantly lower heart rates, both after BII alone or combined with HA (BIII). In an additional experiment, the time needed to "ride" 15 km on a bicycle ergometer was reduced by 10% after FWI as compared to control time.