It is well known that elevations in body temperature can impair both physical and cognitive performance. For helicopter pilots, the major heat source during flight originates from solar radiation. However, when nuclear, biological and chemical (NBC) protective clothing is worn, metabolic heat generated during the pre-flight period, and during the mission, is trapped, exacerbating thermal strain. In this project, the hypothesis that elevations in body heat content would degrade flight performance was tested. Helicopter pilots completed three, two-hour flight simulations under three levels of thermal strain, administered in a balanced order. Thermal strain was induced using a water-perfusion gannent, worn under flight and NBC clothing, and supplied with water that would elicit skin temperatures of 33°C (control), 37°C (moderate) and 39°C (hot). Each sortie was programmed and controlled by the simulator flight officer (blind to treatment order), and was comprised of eight flight circuits, each involving takeoff and landing exercises. During each circuit, the pilots were required to identify and solve two operational problems, graded as "easy", "moderate" and "hard". In this report, we present preliminary data from four pilots. Terminal core temperatures for each trial were: 37.5°C (±0.17; control), 38.6°C (±0.14; moderate) and 38.8°C (±0.19; hot). This strain was also reflected within the terminal heart rates: 87.8 b.min-1 (±9.6; control), 127.8 b.min-1 (±9.1; moderate) and 150.8 b.min-1 (±6.1; hot). The simulator officer independently graded pilot performance, and while the moderate trial resulted in slightly reduced performance scores, relative to control (P>0.05), scores were significantly lower in the hot trials (P<0.05). Strong correlations existed between the thermal load and both the effort needed to sustain the appropriate flight performance, and the pilots' own assessment of performance quality. From these data, it appears that mean body temperature has the most powerful impact upon perceived performance quality, whilst mean skin temperature appears to largely determine effort perception. In both instances, the explained variance currently exceeds 65%.