In experiments on rats, a 14-days blockade of H-,K- -ATPase with lansoprazole was revealed to cause structural disorders in the epitheliocytes and goblet cells, enhancement of proliferation processes in the mucous membrane of large intestine and increase of gastrin concentration in blood. The blockade of H-,K+ -ATPase in epitheliocytes elicited an impairment of their structure, whereas activation of proliferative processes is caused by gastrin action. Morphological changes under the action of lansoprasole may cause disorders in the processes of ion transport, microhemodynamics and activation of proliferative processes. The blockade of CCK-2 gastrin receptors with proglumide resulted in a decrease ofgastrin concentration in blood and reduction oflansoprazole action. Monitoring the blood gastrin levels is preferable to perform with the use of H-,K- -ATPase blockers and under condition of stomach hyposecretion.