The use of secretin to facilitate the demonstration of Giardia lamblia in duodenal juice was studied in children under investigation for chronic diarrhea. 30 children aged 3 mth to 13 yr, whose stools were negative for G. lamblia, were studied. G. lamblia was demonstrable in a sample of duodenal juice in 1 of the 30 children before an i.v. injection of secretin (1 or 2 mg/kg), but in 9 of the 30 children after secretin. It is concluded that examination of duodenal juice after secretin stimulus is an effective method of showing giardial infestation.