The purpose of this study was to examine 1) wildfire fighters’ ability to consume the prescribed fluid volume (1200 mL h(-1)), 2) the effect of fluid intake on plasma sodium and hydration, and 3) the effect of fluid intake on firefighters’ heart rate, core temperature and activity during emergency suppression shifts. METHODS: Thirty-four firefighters were divided into ad libitum (AD, n =17) and prescribed (PR, n = 17) drinking groups. RESULTS: PR drinkers did not meet the prescribed fluid target, yet consumed over double the volume of AD drinkers. No differences between groups in plasma sodium or hydration were noted. PR drinking resulted in lower core temperature between 2 and 6 h. This did not coincide with reduced cardiovascular strain, greater work activity or larger distances covered when compared to AD drinkers. CONCLUSION: Extra fluid consumption (above AD) did not improve firefighter activity or physiological function (though PR fire fighters core temperature was lower earlier in their shift). Firefighter can self-regulate their fluid consumption behavior and work rate to leave the fireground euhydrated.