Introduction: Standby divers must be fully dressed in the appropriate ensemble during military and commercial diving operations. These garments are often fully encapsulating and may result in heat stress when worn in warm environments. This heat stress may lead to hypohydration, which can result in reduced physical and cognitive capacity before the standby diver enters the water. Factors that contribute to heat stress include ambient temperature, the diving ensemble, and radiant heat load from direct sun exposure. Objective: We examined the physiologic responses to heat stress in subjects wearing a Viking HD drysuit during one hour of exposure to dry-bulb temperatures of 33, 36, 39, and 42°C. Methods: Protocols were completed on four different days, and conditions were randomly assigned and delivered in an environmental chamber without radiant heat. Euhydrated subjects donned a heavy rubber drysuit over a lightweight base layer. The drysuit was appropriate for contaminated water diving with integrated boots and neck dam. Subjects also wore chemical protective gloves. Heart rate (HR), core (Tc), and skin (Tsk) temperatures were monitored during the one-hour exposure. Nude body mass was recorded before and after. Two-way ANOVA was used to analyze the data, and significance was set to p £ 0.05. Results: Eight subjects (4 males) aged 27±5 y completed all study conditions. HR and Tc increased over time (p<0.001). Mean seated HR peaked at 138±17 bpm in the 42°C and at 114±13 bpm in the 39°C condition. Peak Tc was 37.0±0.16, 37.1±0.19, 37.4±0.27, and 37.9±0.48 °C in the 33°C, 36°C, 39°C, and 42°C conditions, respectively and differed between all conditions (p<.001) except 33°C and 36°C (p=.60). The temperature gradient from Tc to Tsk was eliminated after 30 min in the 42°C condition and after 40 min in the 39°C. The Tc to Tsk gradient was nearly eliminated in the 36°C at the end of the 60 min exposure. Sweat rates of 0.17±0.09, 0.33±0.18, 0.57±0.4, and 0.82±0.5 L/hr were observed in the 33°C, 36°C, 39°C, and 42°C conditions, respectively and corresponded with a -0.20±0.10, -0.40±0.19, -0.69±0.46, and -0.99±0.55 % change in body mass. Sweat rate in both 39°C and 42°C was higher than 33°C (p<0.001). Conclusions: Even in the absence of radiant heating, significant hypohydration and heat stress occurs in standby divers after 30 min of exposure to 42°C and after 40 min at 39°C. Awareness of the conditions and rotation of standby divers could increase mission safety in these warm environments. Rehydration based on projected sweat rates can also be considered. Simple cooling strategies, such as hand/forearm immersion, could be examined to further manage heat stress in standby divers. Ethical Considerations: This study was approved by the University at Buffalo Institutional Review Board.