Aim of this study was to investigate the effect of geranylgeranylacetone (GGA) on mucus synthesis and secretion in rat gastric cultured cells, and their relationship to prostaglandin (PG) synthesis. Rate of mucus synthesis was estimated by incorporation of 3H-glucosamine into the cultured cells. Release of 3H-glucosamine from the cells, which were preincubated in the medium containing the radioactive isotope, into the culture media was measured for the evaluation of mucus secretion. PG production by the cultured cells was measured by radioimmunoassay. GGA increased glycoprotein synthesis in a dose-dependent manner (p less than 0.01). Secretion of mucus from cultured cells was also significantly enhanced by GGA. GGA did not significantly increase PG (E2 and I2) production. These results indicate that GGA has the ability to stimulate mucus production by the gastric epithelial cells, and this action may play an important role in protective effect of GGA. It is, however, unlikely that this effect of GGA is mediated by endogenous PGs.