Background: The present study was designed to examine whether water temperature during head-out immersion (HOI) modifies hemodilution dynamics. Methods: We made continuous measurements of blood density (rho b) during HOI at 3 different water temperatures; the lower critical (32 degrees C), neutral (34.5 degrees C), and upper critical (36 degrees C) temperatures in 6 healthy male volunteers. Blood was withdrawn continuously from the antecubital vein for measurement of rho b during 60 min of water immersion with a 10-min control period before the immersion. The density was measured with the mechanical oscillator technique. Hematocrit (Hct), plasma density (rho p), and osmolality were measured at 5-min intervals. Erythrocyte density (rho e) was calculated from rho b, rho p and Hct. Cardiac output and BP were measured to calculate total peripheral resistance. Results: Hct, rho h, and rho p decreased rapidly in the first 20-25 min of immersion and were maintained at a reduced level during immersion. Plasma volume calculated from rho p and Hct increased with the rho b reduction. These immersion-induced changes were independent of these water temperatures. Plasma osmolality and rho e remained constant throughout the experimental period in the three temperature conditions, indicating that the increase in plasma volume and hence hemodilution was induced by an isotonic fluid shift from extravascular space. The total peripheral resistance increased inversely in proportion to the water temperature during HOI. Conclusion: In the present condition, water temperature did not modify the net transcapillary fluid transfer during HOI in the presence of the temperature dependent increase in vascular tone.