During a whole body heat stress, stroke volume is either maintained or slightly elevated despite reduced ventricular filling pressures and central blood volume, suggestive of improved cardiac diastolic and/or systolic function. Heat stress improves cardiac systolic and diastolic function in patients with congestive heart failure, although it remains unknown whether similar responses occur in healthy individuals, which is the hypothesis to be tested. Nine male volunteers underwent a whole body heat stress. Echocardiographic indexes of diastolic and systolic function were performed following a supine resting period, and again following an increase in internal temperature of approximately 1.0 degrees C via passive heat stress. Despite previous reports of heat stress-induced decreases in ventricular filling pressures and central blood volume, no changes in indexes of diastolic function were identified during heating [i.e., unchanged early diastolic mitral annular tissue velocity (E'), mitral inflow during the early diastolic phase (E), the E/E' ratio, and isovolumetric relaxation time]. Heat stress increased late diastolic septal (P = 0.03) and lateral (P = 0.01) mitral annular tissue velocities (A'), mitral inflow velocity during atrial contraction (P < 0.001), and the relative contribution of atrial contraction to left ventricular filling during diastole (P = 0.01), all indicative of improved atrial systolic function. Furthermore, indexes of ventricular systolic function were increased by heat stress [i.e., increased septal (P = 0.001) and lateral (P = 0.01) mitral annular systolic velocities and isovolumic acceleration at the septal (P = 0.03) and lateral (P < 0.001) mitral annulus]. These data are suggestive of improved atrial and ventricular systolic function by the heat stress. Together these data support previous findings, which used the less precise measure of ejection fraction, that heat stress improves indexes of systolic function, while diastolic function is maintained.