To investigate whether prostaglandins (PGs) are involved in the mechanism of rotavirus diarrhea, PGE2 and PGF2 alpha concentrations in the plasma and stool of children (21 and 16, respectively) with rotavirus gastroenteritis were measured and compared with those of their respective controls. The effect of aspirin on the diarrhea was also studied in 14 patients with rotavirus gastroenteritis. The PGE2 and PGF2 alpha contents of both the plasma and the stool of patients with rotavirus gastroenteritis were significantly higher than in the plasma and stool of the control group. Furthermore, aspirin given by mouth caused the diarrhea to cease earlier. These results suggest that PGE2 and PGF2 alpha are involved as important mediators in the causative mechanism of rotavirus diarrhea.