This study was designed to test whether concentration or dose of NaCl was responsible for the initial tissue damage (after 1 min) and resulting temporary cell proliferation at 17 h in stomach mucosa of male F344 rats after gastric intubation of 0.65, 1.3, 2.6 and 3.7 M NaCl. Histological damage was studied by dual staining combining horseradish peroxidase-labeled Griffonia simplicifolia agglutinin-II staining (HRP-GSA-II) and periodic acid cold thionin-Schiff reaction (PATS). Cell proliferation was studied by measuring replicative DNA synthesis with liquid scintillation counting and by BrdU staining. NaCl at the same overall dose of 0.8 g/kg body weight induced different degrees of response depending on the concentration. For 4 ml of 0.65 M NaCl, there was no tissue damage after 1 min nor any increase in replicative DNA synthesis after 17 h in the pyloric mucosa. Administration of 1.3 M NaCl (2 ml), 2.6 M NaCl (1 ml) and 3.7 M NaCl (0.7 ml) induced concentration-dependent damage of the surface mucous cell layer after 1 min and increased replicative DNA synthesis after 17 h (P<0.05). Concentration-dependent increase in replicative DNA synthesis at 17 h was also induced with the same volume (1 ml) of 1.3, 2.6, and 3.7 M NaCl, while a volume-dependent increase in replicative DNA synthesis at 17 h was induced with 0.4, 0.7 and 1 ml of 3.7 M NaCl. However, a greater increase in replicative DNA synthesis was always observed when using higher NaCl concentrations at the same dose. Liquid scintillation counting was well-correlated with BrdU staining. These results suggest that a high concentration of NaCl is responsible for the initial tissue damage and resulting temporary cell proliferation during stomach tumor promotion.