PURPOSE: To assess whether ad libitum water ingestion of different temperatures is sufficient to prevent dehydration-related exacerbations of thermal and cardiovascular strain, during exposure to conditions representative of a heatwave. METHODS: Twelve participants (mean +/- SD; 25 +/- 4 years) exercised for 180 min at 3 METs in 40.1 +/- 0.6 degrees C, 40.4 +/- 2.1%RH four times: (i) consuming 20 degrees C water ad libitum (AL20); (ii) consuming 4 degrees C water ad libitum (AL4); (iii) replacing no fluids (NOFR); (iv) replacing sweat losses (FULLFR). Fluid consumption (FC), dehydration (%DEH), rectal temperature (Tre), rate-pressure product (RPP), forearm blood flow (FBF), mean skin temperature (Tsk), and local sweat rate (LSR) were measured/determined. RESULTS: FC was greater in AL20 (1.30 +/- 0.41 L) than AL4 (1.03 +/- 0.32 L; P = 0.003). %DEH was lower (P < 0.001) in AL20 (0.11 +/- 0.76%), AL4 (0.43 +/- 0.64%), and FULLFR (0.01 +/- 0.12%) compared to NOFR (1.93 +/- 0.28%). %DEH was lower in AL20 than AL4 (P = 0.003). In NOFR, end-trial changes in Tre were greater (P < 0.001) (1.05 +/- 0.27 degrees C) compared to all other trials, but similar among AL20 (0.72 +/- 0.30 degrees C), AL4 (0.76 +/- 0.25 degrees C) and FULLFR (0.74 +/- 0.35 degrees C). End-trial RPP was higher (P < 0.001) in NOFR (12,389 +/- 1578 mmHg.bpm) compared to all other trials, but similar among FULLFR (11,067 +/- 1292 mmHg.bpm), AL20 (11,214 +/- 2078 mmHg.bpm) and AL4 (11,089 +/- 1795 mmHg.bpm). No differences in Tsk or LSR were observed among trials, but FBF was lower in NOFR compared to FULLFR (2.84 +/- 0.69 vs. 3.52 +/- 0.96 ml/100 ml/min; P = 0.029). CONCLUSION: 4 degrees C or 20 degrees C ad libitum water ingestion prevented dehydration levels that exacerbate thermal/cardiovascular strain, despite blunted fluid intake with 4 degrees C water. Higher core temperatures with NOFR are attributed to impaired internal heat distribution secondary to a lower FBF.