The possibility that the decreased urinary flow during continuous positive-pressure breathing (CPPB) may be a consequence of a reflex mediated via the cardiopulmonary baroreceptors to increase neurohumoral secretion or to change the sympathetic outflow was assessed. Muscle sympathetic nerve activity (MSNA) on the right peroneal nerve, vasoactive hormones, and renal and cardiovascular responses were measured during CPPB (+12 mmHg) in 10 male subjects (22.0 +/- 0.6 yr, 66.8 +/- 1.5 kg body wt). The experiments consisted of a 1-h control, 1 h with CPPB (experimental) or without CPPB (a time control), and a 1-h recovery period. Two blood samples were taken during each period for measurements of arginine vasopressin (AVP), plasma aldosterone (PAldo), plasma renin activity (PRA), norepinephrine, and atrial natriuretic peptide (ANP), and urine was collected hourly for the measurement of urine volume and electrolytes and clearances. MSNA rapidly increased (P < 0.05) at the onset of CPPB, continued to increase during exposure, and rapidly returned to the normal level at recovery. The MSNA changes coincided with increased plasma NE and were concurrent with a reduced (P < 0.05) urine output associated with a reduction of both free water and osmolal clearances, Na+ and osmolal excretions, and creatinine clearance (glomerular filtration rate). AVP and PRA increased (P < 0.05), whereas PAldo and ANP were unchanged. The results are consistent with the concept that increased sympathetic outflow may play a role in the reduction of urinary output and Na+ excretion during unloading of the cardiopulmonary receptors.