To evaluate the separate cardiovascular response to body immersion and increased environmental pressure during diving, 12 healthy male subjects (mean age 35.2+6.5), underwent 2D-Doppler Echocardiography in 5 different conditions: out of water (BASAL); head-out immersion while breathing (A); fully immersed at the surface while breathing (B) and breath-holding (C); breath-hold diving at 5m depth (D). Heart rate, left ventricular volumes, stroke volume and cardiac output were obtained by underwater echocardiography. Early (E) and late (A) transmitral flow velocities, their ratio (E/A) and deceleration time of E (DTE) were also obtained, from pulsed-wave Doppler, as left ventricular diastolic function indexes. The experimental protocol induced significant reductions in left ventricular volumes, left ventricular stroke volume (P<0.05), cardiac output (P<0.001) and heart rate (P<0.05). A significant increase in E peak (P<0.01) and E/A ratio (P<0.01) and a significant reduction of DTE (P<0.01) were also observed. Changes occurring during diving (D) accounted for most of changes observed in the experimental series. In particular, cardiac output at D resulted significantly lower as compared with each other experimental condition, E/A ratio resulted significantly higher during D than in A and C. Finally, DTE resulted significantly shorter at step D than in BASAL and C. This study confirms a reduction of cardiac output in diving humans. Since most of changes were observed during diving, the increased environmental pressure seems responsible of this hemodynamic rearrangement. Left ventricular diastolic function changes suggest a constrictive effect on the heart possibly accounting for cardiac output reduction.