The effect of the thiamine deficiency on the H+ secretion in an isolated gastric mucosa and also on the functional properties of the mitochondria was investigated. The thiamine deficiency was called forth through introduction of oxythiamine or by exclusion of thiamine from the ration. It is shown that the thiamine deficiency in an isolated gastric mucosa of the frog produced inhibition of the H+ secretion and in the gastric mitochondria of rats--a higher activity of succinate-dehydrogenase and a diminution of the exogenous cytochrome oxidation rate.