The heat strain produced by three different Canadian Forces chemical defence (CD) individual protection ensembles (IPE) was studied under simulated hot cockpit conditions with an air-cooling vest (AC) and with no cooling (NC). Seven healthy males were randomly subjected to six heat stress tests (37°C, 50% r.h., target of 150 min) using the helicopter IPE with AC-4 mask (H4), the helicopter IPE with AR-5 hood/respirator (H5), and the CF-18 fighter IPE (F) with AR-5 respirator and anti-G suit. Whatever the IPE, AC increased heat exposure time and total heat losses, and decreased the change in core (δT^re/h) and whole body mean skin temperatures (δ¯T^sk(WB)/h) (P<0.05). Differences within IPEs appeared restricted to the F-NC condition. F-NC produced a lower heat exposure time (vs H4-NC and H5-NC) and sweat evaporation rate (vs H4-NC), and a greater δ¯T^sk(WB)/h (vs H4-NC and H5-NC) and T^re at min 80 (vs H5-NC) (P<0.05). The results demonstrate that air cooling greatly enhances heat tolerance of subjects wearing any of the 3 CD IPEs tested. They also indicate that without cooling, both helicopter IPEs produce slightly better tolerance to heat than the fighter IPE, possibly an influence of the anti-G suit on evaporative cooling.