Aspirin reduced plasma concentrations of prostaglandin (PG) F alpha in 11 of 12 children with toddler diarrhoea, and usually, but not always, controlled the symptom. Loperamide consistently controlled toddler diarrhoea in 10 patients but had no effect on plasma PGF alpha. In eight patients experiencing spontaneous remission of symptoms, plasma PGF alpha was significantly lower than during diarrhoeal episodes. These results suggest that (a) toddler diarrhoea is in some cases mediated by PG, and (b) the effect of loperamide is independent of PG levels.