The effect of cimetidine on normal human gastric mucus has been compared with that of carbenoxolone, a drug believed to enhance mucus production. Each drug was given for two weeks, the gastric contents aspirated over a timed period and the results assessed in unstimulated and pentagastrin stimulated secretions. The volume, dry weight and the carbohydrate contents of non-diffusable glycoconjugates, high molecular mass glycoproteins and glycopolypeptides were investigated. Both drugs reduced the volumes of stimulated secretions. This was statistically significant after cimetidine. More importantly neither drug affected the amount of non-diffusable glycoconjugates, so that the concentration remained the same or increased. Both drugs reduced the monosaccharide content of the high molecular mass fractions. This reached significance for the stimulated secretion after cimetidine. As the carbohydrate content of the glycopolypeptides was unchanged this indicated the presence of a non-mucin glycoprotein or protein. Overall there was no fundamental difference between the results for cimetidine and carbenoxolone.