The present study describes our observations on optical and ultrastructural features of serotonin-containing cells in the rat antral and upper duodenal mucosa, utilizing optic morphometric measurements in a model of experimental chronic alcoholism of rat in which nutrition was well controlled. Male Wistar rats were given ethanol to provide 23 per cent of the total calories, while starch replaced ethanol isocalorically in controls. Twenty-five per cent of the calories were provided by protein in both groups. Blood levels of serotonin were significantly raised after chronic ethanol feeding (0.059 +/- 0.06 vs. 0.159 +/- 0.012 micrograms/ml, p < 0.01). Decrease in the number of immunohistochemically-detectable serotonin-containing cells was found in the pyloric gland mucosal area specimens of the chronically ethanol-treated rats (68.9 +/- 5.2 vs 43.3 +/- 3.0; p < 0.001). The immunohistologically-evaluated number of the same cells in the duodenal mucosa specimens was significantly decreased by alcohol feeding. Although total villi and crypt count per whole circular section, and the number of crypts per villus were not significantly changed either in control animals or in chronically ethanol-fed rats, decreased number of these cells per whole circular section (289 +/- 21.6 vs. 183 +/- 10.5; p < 0.001) per villus (2.52 +/- 0.14 vs. 1.21 +/- 0.10; p < 0.001) and per crypts (0.97 +/- 0.08 vs. 0.79 +/- 0.04; p < 0.05) were reported after alcohol consumption. In both control and experimental rats the cells were predominantly found in the basal half of the antropyloric mucosa. Alcohol did not lead to any changes in normal distribution of the duodenal serotonin-producing cells. The above quantitative changes in serotonin-producing cells were not accompanied by changes in their subcellular appearance in stomach and duodenal mucosa of alcohol-treated rat.