We studied the anticholinesterase activity of three H2‐receptor antagonists (cimetidine, ranitidine, and famotidine) in vitro and in conscious dogs with chronically implanted strain‐gauge force transducers. In vivo, acetylcholine (ACh) was infused intravenously at a dose of 0.05 mg/(kg · min) for 5 minutes with or without a background continuous intravenous infusion of H2‐receptor antagonists or neostigmine during the quiescent period of the interdigestive state. Cimetidine and ranitidine enhanced the ACh‐induced contractions in a dose‐dependent manner in the gastric antrum, whereas famotidine did not. In vitro, the median inhibitory concentration (IC50) of the acetyl‐cholinesterase activity of ranitidine was 3.5 × 10−6 M, and that of cimetidine 2.5 × 10−4 M, whereas famotidine had no effect on cholinesterase activity even at concentrations up to 10−3 M. The effects of a bolus intravenous injection of the three H2‐receptor antagonists on gastric motor activity also were examined in the digestive state. Cimetidine at 10.0 mg/kg and ranitidine at 3.0 mg/kg significantly increased gastric motor activity. This dose of ranitidine, however, sometimes caused the dogs to collapse and significantly decreased blood pressure in the anesthetized dogs. In conclusion, the H2‐receptor antagonists cimetidine and ranitidine enhanced gastric motor activity through the mechanism of their anticholinesterase activity, but further studies on gastric emptying and the circulatory system are needed.