We studied the effect of the specific H2 receptor agonist dimaprit on gastric emptying using a dye dilution technique in five chair-adapted rhesus monkeys to determine simultaneously gastric emptying and H+ secretion. Continuous subcutaneous injections of either saline alone or dimaprit (15, 30, 60, 120, and 240 nmol.kg-1.min-1) were given in random order on separate days. Both fractional emptying rate and H+ output were significantly increased by higher doses of dimaprit, with resulting twofold concurrent increase of intragastric H+ concentration. Because intragastric administration of H+ decreases gastric emptying and secretion, we evaluated the possibility that the stimulatory action of a histamine H2 agonist was underestimated when gastric pH was greater during control than after dimaprit. When exogenous HCl was added to the stomach to achieve H+ concentrations similar to those measured after dimaprit, both fractional emptying and H+ secretion were decreased twofold. When these "acidified control values" were used as a point of comparison, the stimulatory effect of dimaprit on both fractional emptying and gastric secretion was greater, suggesting that the stimulatory actions of dimaprit are underestimated if one does not take into account the differences in intragastric H+ concentrations.