Intestinal function in mallard ducklings (Anas platyrhynchos) on a freshwater regime was studied after a 7-day dietary ingestion of 0.25% and 2.5% Prudhoe Bay crude oil (PBCO) or a 2.5% paraffin mixture with an in vivo luminal perfusion technique. Dietary ingestion of 2.5% PBCO may have an effect on the integrity of the duckling intestine. There were no significant effects of PBCO on the absorption of Na, Cl, K, or H2O compared with control animals. The ducklings fed 2.5% paraffin had a significant depression in Na and H2O absorption compared with controls. However, this depression dose not seem to be related to an effect on intestinal mucosa Na-K-ATPase activity.