Heat strain for six young, healthy, acclimatized men (mean age 26.2 yrs., weight 84.1 Kg) was measured during moderate exercise at various ambient conditions (21.5 C, 28.0 C, 31.5 C with sunshine) while wearing fully encapsulating chemical protective suits with SCBA. The total weight of the protective ensemble was 26.3 Kg. The subjects performed a total of 35 minutes (20 minutes exercise, 5 minutes rest, 15 minutes exercise) of zero grade walking on a treadmill set for 4.83 Kph (3.0 mph). The average level of energy expenditure for this exercise, determined from V(02) measurements was 383 Kcal/hr. Heart rate and mean skin temperature rose significantly as ambient temperature increased. Under the most adverse ambient conditions (31.5 C with sunshine) the mean heart rate and skin temperature were elevated 39.6 bpm and 4.1 C, respectively, over those recorded for control conditions. Significant increases in rectal temperature were not noted. A mean difference in weight loss was only observed with significance between control conditions and the most severe ambient environment (31.5 C with sunshine). The five minute recovery heart rate (SMRHR), recorded at minute 25 after 20 minutes of exercise increased significantly as ambient conditions become more adverse. The mean 5MRHR were 91.7 bpm (control), 95.8 bpm (21.5°C), 108.7 bpm (28°C), and 116.4 bpm (31.5 C with sunshine). It is concluded that wearers of impermeable protective clothing show progressive increases in heat strain as ambient temperature increase. This study indicates recovery heart rate is probably the best indicator of heat tolerance endpoints for work in encapsulating, impermeable protective clothing. Recovery heart rates are easily measured with inexpensive equipment. More study is required, however, before specific recovery heart rates can be identified as a conservative endpoint.