The effect of esaprazole, a new antiulcer compound endowed with gastro-protective properties, on rat gastric mucus was investigated after acute oral administration. Both soluble (luminal) and insoluble (parietal) mucus were examined for their content of: acidic and neutral glycoproteins; N-acetylneuraminic acid by colorimetric techniques; and fucose by HPLC methods. One hour after dosing (50-200 mg/kg) a 2-15-fold increase in soluble mucus and a 2-4-fold increase in insoluble mucus output were observed. Under the same experimental conditions, carbenoxolone (200 mg/kg, p.o.) caused a similar increase in gastric mucus output. These results demonstrate that quantitative changes in both luminal and parietal mucus occurred after orally dosed esaprazole: it is tempting to speculate that this effect might be responsible, at least in part, for the gastro-protective action of the drug.