Left ventricular (LV) systolic function increases with passive heat stress (HS); however, less is known about diastolic function. Eight healthy subjects (24.0 +/- 2.0 yr of age) underwent whole body passive heating approximately 1 degrees C above baseline (BL). Cardiac magnetic resonance imaging was used to measure biventricular volumes, function, filling velocities, volumetric flow rates, and LV twist and strain at BL and after 45 min of HS. Passive heating reduced left atrial volume (-17.6 +/- 11.7 ml, P < 0.05), right and LV end-diastolic volumes (-22.7 +/- 11.0 and -25.7 +/- 24.9 ml, respectively; P < 0.05), and LV stroke volume (-6.7 +/- 6.8 ml, P < 0.05) from BL. LV ejection fraction (EF), end-systolic elastance, septal and lateral mitral annular systolic velocities, circumferential strain, and peak LV twist increased with HS (P < 0.05). Right ventricular stroke volume, EF, and systolic tissue velocities were unchanged with HS (P > 0.05). Early LV diastolic tissue and blood velocities and strain rates were maintained with HS, whereas untwisting rate increased significantly from 166.4 +/- 46.9 to 268.7 +/- 76.8 degrees /s (P < 0.05). The major novel finding of this study was that, secondary to an increase in peak LV twist and untwisting rate, early diastolic blood and tissue velocities and strain rates are maintained despite a reduction in filling pressure.