We made direct measurements of the thickness of the layer of insoluble gel mucus lying on the gastric mucosa by using a slit lamp and an image-splitting system known as a pachymeter. Under nonstimulated conditions, the thickness (mean +/- SE) of the gastric gel mucus on fundic mucosa was 166 +/- 10 micrometers in the rat, 234 +/- 9 micrometers in the guinea pig, 429 +/- 17 micrometers in the dog, and 576 +/- 81 micrometers in human stomach. Using a pylorus-ligation rat model, the effects of distention and of topical application of acid, 16,16-dimethyl prostaglandin E2 and carbenoxolone, on mucus gel thickness were studied. Distention of the stomach produced an increase in gel mucus thickness that was correlated with the degree of distention. This distention effect was not affected by pretreatment with methscopolamine or indomethacin. Topical application of 0.1 M HCl had no demonstrable effect on gel mucus thickness when compared with phosphate buffer, pH 7.4. 16,16-Dimethyl prostaglandin E2 applied to the mucosa in concentrations of 1 microgram . ml-1 and 10 microgram . ml-1 caused an increase in gel mucus thickness of 81% and 140%, respectively, and carbenoxolone, 2.5 mg . ml-1, an increase of 78%. These studies suggest that thickness of gel mucus overlying the gastric mucosa is dynamic, being subject to increase or decrease under certain physiologic and drug-treatment conditions.