The effect of long-term administration of an antacid preparation on the gastric mucosa was investigated in rats, special attention was directed towards hypergastrinaemia and density of argyrophil cells. One ml of an Al (OH)3- and Mg (OH)2-containing antacid (in vitro neutralization capacity 28 mmol/die) or water was administered intragastrically 4 times daily. An additional group of rats remained untreated. Twelve hours after the final dose serum gastrin levels were significantly (p less than 0.001) elevated (113 +/- 28 pg/ml) compared to the control groups (33 +/- 1 and 22 +/- 2 pg/ml). Antral gastrin (G)-cell density was also increased after antacids by 58% whereas the somatostatin (D)-cell density and the somatostatin concentration in antral tissues were decreased. The number of fundic D- and argyrophil cells were not altered by antacid treatment. The number of parietal cell declined significantly in response to antacids. The foveolar gland region was almost doubled after antacids. It is concluded that in the rat 1. despite persistent hypergastrinaemia due to chronic antacid administration increases in argyrophil cell densities are not to be found; 2. long-term administration of antacids exert a trophic effect on the corpus mucosa predominantly by an increase of mucus neck cells.