This research was carried out to define the effects on man during head-out water immersion in a bath at 38.41 +/- 0.04 degrees C (mean +/- S.E.) with a method similar to that used for therapeutical rehabilitation and time of immersion of 30 minutes. Hemorheological, hematic and hemodynamic parameters were analysed. Seventeen healthy subjects (fourteen males and three females), between the ages of 21-65 years and mean age of 29.8 +/- 2.6 years were studied. Head-out water immersion resulted in: 1) decrease in blood viscosity, red blood cells count, C-Hct and M-Hct, without significant changes in leukocytes and platelets count, MCV, plasma viscosity, erythrocyte filtration time and RCDI; 2) an increase in heart rate and a decrease in systolic and diastolic blood pressure. Thirty minutes after the end of immersion, heart rate, diastolic blood pressure and blood viscosity, measured at 0.512 sec-1 shear rate, returned to pre-immersion values; systolic blood pressure showed a slight increase but was still significantly below the basal levels; erythrocytes count, C-Hct, M-Cct and blood viscosity, measured at 94.5 sec-1 shear rate, significantly exceeded pre-immersion values. The probable pathogenesis of these observations is suggested. A matter of great interest is the study of the same parameters in elderly subjects, with or without cardiovascular diseases, or in patients using drugs affecting blood pressure, blood viscosity or hemocoagulation process.