I An isolated stomach preparation from immature rats is described. The lumen of the stomach was perfused and the hydrogen ion activity of the perfusate recorded continuously. 2 The preparation gave dose-dependent responses to gastrin, acetylcholine and dibutyryl cyclic adenosine 3',5'-monophosphate and these responses were readily reversed on washing out the agonist. 3 The acid secretory response to gastrin was inhibited by metiamide at concentrations of 10-M and 3 x 1O-5M. 4 The acid secretory responses to acetylcholine and dibutyryl cyclic adenosine 3',5'-monophosphate were not inhibited by concentrations of metiamide up to 103 M. 5 These findings are discussed in relation to the role of histamine in the control of gastric acid secretion.