We examined the effects of Z-300 (N-[3-[3- (piperidinomethyl)phenoxy]propyl]-2-(2-hydroxy-ethyl-1-thio)acetam ido.2- (4-hydroxy benzoyl)benzoate), a newly-synthesized selective histamine H2-receptor antagonist, on mucin in rat gastric mucosa. Deep corpus mucin content increased significantly to 127% of the control after the administration of 30 mg/kg of Z-300, whereas that in the antral mucosa did not increase. The addition of Z-300 significantly increased [3H]-labeled mucin in the corpus region. In the antrum, biosynthetic activity showed no significant change by 10(-8)-10(-5) M of Z-300. These results suggest that Z-300 not only inhibits acid secretion but may also promote gastric mucus metabolism in the corpus region.