Thermoneutral head-out water immersion (WI) of humans leads to a sustained elevation of cardiac output (QCO), while systemic VO2 is unchanged. However, the regional blood flow responses (Qr) to WI are largely unknown. In eight dogs anesthetized with pentobarbital sodium, relative to the control vertical position in air with the chest and abdomen upright (Erect1), after 30 min of WI at 38 degrees C the QCO (thermodilution), stroke volume, and aortic pressure were elevated (P less than 0.01) by 88.5 +/- 6.4% (SE), 55 +/- 9%, and 29 +/- 4% from levels of 2.24 +/- 0.01 1 X min-1, 18.4 ml +/- 1.7, and 109 +/- 7.9 mmHg, respectively. Qr (ml X min-1 X 100 g-1), measured by 15 microns-radiolabeled microspheres at WI30 min relative to the Erect1 levels, increased (P less than 0.01) in the stomach (104 +/- 27%), jejunum (78 +/- 19%), ileum (102 +/- 23%), liver (hepatic artery) (485 +/- 46%), pancreas (200 +/- 44%), spleen (88.7 +/- 33%), renal cortex (88 +/- 30%), left ventricle (157 +/- 15%), skin (726 +/- 30%), fat (680 +/- 43%), gastrocnemius (200 +/- 36%), triceps (414 +/- 61%), intercostals (317 +/- 44%), and diaphragm (477 +/- 24%). Qr responses at WI10 min were largely similar. Rectal temperature and VO2 were unchanged during WI. In the lateral recumbent position in air, QCO rose by 44.3% (P less than 0.01), but only hepatic arterial Q rose significantly (89 +/- 29%). Therefore, there may be a partial uncoupling of the normal Q/VO2 relationship in WI.