Pulmonary capillary blood volume (Qc) was determined for 7 subjects in the standing posture and immersed up to the sternal manubrium at three water temperatures: 34 degrees C +/- 0.5 degrees C, thermally neutral bath; 25 degrees C +/- 0.5 degrees C, cold bath; and 40 degrees C +/- 0.5 degrees C, hot bath. The Qc was calculated from the lung transfer factor DLco measured while breathing two gas mixtures (21.1% O2 and 90.0% O2) during breath holding. Control experiments in a dry air environment show that Qc values for standing posture decrease compared to the sitting values, owing to a redistribution of the intrathoracic blood volume to lower body parts as a result of gravity. Immersion at 34 degrees C in an upright position produces a significant increase in Qc (P less than 0.01). This is a result of the hydrostatic counterpressure: blood shifts from the periphery to the intrathoracic regions. Immersion at 25 degrees C increases Qc compared to the values obtained at 34 degrees C, but the difference is not significant. The contribution of vasoconstriction to blood volume shift in cold water is probably less important than that of hydrostatic counterpressure. During immersion at 40 degrees C, the rise in Qc is very significant (P less than 0.05). This may be explained by an increase in cardiac output and central blood volume when skin temperature is raised at 40 degrees C.