The present investigation was aimed to find out the impact of the diuretics on the histological structures of the gingiva and tongue mucous membranes and the immunohistochemical localization of the proliferating cell nuclear antigen in the aforementioned structures. Thirty adult male albino rats with average body weight 150 ± 10 grams were divided into five groups, 10 animals each. Group I, was served as controls were animals received in group II Acetazolamide (Diamox) in a daily oral dose of 8 mg. Group III animals received hydrochlorothiazide (Esidrix) in a daily oral dose of 50 mg. Group IV animals received Amiloride (Midamor) in a daily oral dose of 4 mg, furosemide (Lasix) in a daily oral dose of 40 mg in group V. The specimens were processed to be embedded in paraffin and stained with Hematoxylin and eosin for the mucous membrane of gingiva and tongue, Masson's trichrome stain for the demonstration of collagen fibers of the laminapropria of the mucous membranes of the gingiva and tongue, with PCNA mouse monoclonal antibody for the proliferating cell nuclear antigen.