We examined changes in the apparent whole-blood viscosity (aWBV) of healthy subjects using non-anticoagulated specimens during Japanese style bathing. We have developed a falling needle rhemeter (FNR) that is able to measure several terminal velocities with resin needles in various densities within 210 s. When a needle falls into whole blood in the columnar container, the parameters of aWBV, shear stress and shear rate can be calculated using the blood density, needle density and terminal velocity of the needle as measured by the FNR. Multiple measurements can be made in one whole-blood fluidity analysis, using only a small amount of specimen without any anticoagulant. In this study, several non-eldery healthy adults remained immersed up to the armpits in a sitting posture in the bathtub. In order to maintain a constant bath temperature, heated tap water kept being added. Whole-blood fluidity was analyzed with several aWBVs at various shear rates measured using the FNR. Referring to a previous study reporting on conditions of hemodynamic change, our preliminary examination found that observation of a change in blood fluidity required 10 min of bathing in water at 42 degrees Celsius. Eight healthy men and one healthy woman therefore took a single-bath for 10 min at 42 degrees Celsius for the single-bathing examination. Six of these nine subjects, including the female subject, showed increased aWBVs in the high-shear-rate region with a single bath. And one subject showed increased aWBVs in the high-shear-rate region, 10 min after bathing. These subjects showed altered whole-blood fluidity in the low-shear-rate region at the time of changing aWBVs in the high-shear-rate region. Six healthy men were enrolled in a double-bathing examination, first bathing for 10 min, then taking a break for 5 min and taking a second bath for 5 min. All subjects showed increased aWBVs in the high-shear-rate region: two of the six enrolled subjects showed increases after the first bath; three subjects showed increases after the second bath; and the remaining subject showed an increase at 10 min after the second bath. All subjects showed increased aWBVs in the high-shear-rate region, and a simultaneous change in whole-blood fluidity in the low-shear-rate region. Notably, the time needed for a change in whole-blood fluidity was very short. To implement the present study, non-elderly adult volunteers were enrolled to reduce the risk of accidents and some limitations were placed on bathing conditions. As changes in blood fluidity were observed under bathing conditions with water temperature over 42 degrees Celsius and immersion up to the armpits in a sitting posture, we concluded that prolonged Japanese-style bathing in water exceeding 42 degrees Celsius is dangerous.