In six human volunteers we studied the effects of hypovolemia on the secretory activity of the gastric mucosa. The secretion of acid and HCO3 from the stomach was calculated from continuous measurements of pH and Pco2 in gastric effluent. Gastric secretion was stimulated by sham feeding (SF), and cardiac filling pressure was decreased by pooling blood in the lower extremities (lower body negative pressure (LBNP]. LBNP at -20 mmHg had no significant effects on systemic arterial pressure or heart rate but increased plasma norepinephrine concentration by 48 +/- 6% (p less than 0.001). Both the acid and the alkaline responses to SF were significantly attenuated during LBNP (-38 +/- 8%, p less than 0.01, and -55 +/- 14%, p less than 0.05, respectively). Analysis of the relationship between acid and HCO3 secretion in individual experiments suggested a relatively more pronounced inhibition of HCO3 secretion. The results imply that a decreased responsiveness of the gastric mucosa may be one component of the cardiovascular reflex adaptation to hypovolemia. A downregulation of active secretion will lead to smaller metabolic demands from the secreting cells and may thereby help to maintain a vasoconstriction in the gastric mucosa.