When castor oil was administered by gavage to rats, the duodenum and jejunum, but not the stomach, produced large amounts of platelet activating factor 3-7 h after oil challenge with a peak at 3 h. Intraluminal release of acid phosphatase was also markedly increased in the duodenum and jejunum of castor oil-treated rats, especially 3-5 h after oil challenge. No increase was observed in the stomach. There was a correlation between elevated release of acid phosphatase and intestinal hyperaemia.