The ability of hypnosis to both stimulate and inhibit gastric acid secretion in highly hypnotizable healthy volunteers was examined in two studies. In the first, after basal acid secretion was measured, subjects were hypnotized and instructed to imagine all aspects of eating a series of delicious meals. Acid output rose from a basal mean of 3.60 +/- 0.48 to a mean of 6.80 +/- 0.02 mmol H+/h with hypnosis, an increase of 89% (p = 0.0007). In a second study, subjects underwent two sessions of gastric analysis in random order, once with no hypnosis and once under a hypnotic instruction to experience deep relaxation and remove their thoughts from hunger. When compared to the no-hypnosis session, with hypnosis there was a 39% reduction in basal acid output (4.29 +/- 0.93 vs. 2.60 +/- 0.44 mmol H+/h, p less than 0.05) and an 11% reduction in pentagastrin-stimulated peak acid output (28.69 +/- 2.34 vs. 25.43 +/- 2.98 mmol H+/h, p less than 0.05). We have shown that different cognitive states induced by hypnosis can promote or inhibit gastric acid production, processes clearly controlled by the central nervous system. Hypnosis offers promise as a safe and simple method for studying the mechanisms of such central control.