Effects of i.v. somatostatin on the gastric mucus output during a 60 min infusion of pentagastrin were examined in five healthy subjects. Experiments were also made with prior indomethacin administration to suppress the synthesis of endogenous prostaglandins. N-acetyl neuraminic acid, a sialic acid, was measured in the gastric aspirates as an index of gastric mucus. Somatostatin increased significantly the gastric mucus output in a dose-related way. The output increased to 158 +/- 14%, and 216 +/- 24% of the control level by 1 and 2 microgram kg-1h-1 somatostatin respectively. The increase induced by somatostatin was prevented by prior indomethacin. With indomethacin alone the mucus output was not different from in controls. Thus somatostatin has augmentory effects on the gastric mucus, which can be blocked by indomethacin, indicating that endogenous prostaglandins are involved in the augmentation. Somatostatin prevents stress ulcerations in the rat, assumedly by inhibiting the gastric acid secretion. This study suggests that additional mechanisms may contribute to the protective action of somatostatin on the gastric mucosa.