The purpose of the present study was to quantify and compare the heat strain when a new chemical defense (CD) vapor protective layer was worn under combat clothing. There were 23 unacclimatized males (27.1 +/- 5.6 y, 80.9 +/- 9.3 kg, 1.78 +/- 0.05 m) assigned to a light intermittent (L, n = 7), moderate continuous (M, n = 8) or heavy continuous (H, n = 8) exercise group. All subjects performed three trials at 40 degrees C and 30% relative humidity wearing the current CD protective clothing over combat clothing (CD-current), the CD vapor protective layer alone (CD-vapor) or under combat clothing (CD-vapor/combats). Tolerance times were significantly different among the three clothing trials for each exercise group. For group L, tolerance time increased from 116 min for CD-current to 208 min for CD-vapor/combats and to 289 min for CD-vapor. For group M, tolerance time increased from 67 to 87 and to 133 min for CD-current, CD-vapor/combats and CD-vapor, respectively. Finally for group H, respective times for CD-current, CD-vapor/combats and CD-vapor were 50 min, 59 min and 80 min. Significant differences among the clothing configurations were found also for the evaporative efficiency of sweat from the clothing, heart rates, rectal and skin temperatures. In all conditions, the response during CD-vapor/combats was between CD-current and CD-vapor. Although heat strain was reduced compared with CD-current, the improvements with CD-vapor/combats might be considered significant in an operational environment only at low metabolic rates.(ABSTRACT TRUNCATED AT 250 WORDS)