Ducklings given hypertonic saline drinking water show significant increases in the rates of Na+ and water transfer across the intestinal mucosa. These increased rates of transfer are maintained as long as the birds are fed dypertonic saline. Oral administration of a single small dose of crude oil had no effect on the basal rate of mucosal transfer in freshwater-maintained ducklings but the adaptive response of the mucosa is suppressed in birds given hypertonic saline. When crude oils from eight different geographical locations were tested, the degree of inhibition varied between them; the greatest and smallest degrees of inhibition being observed following administration of Kuwait and North Slope, Alaska, crude oils respectively. The effects of distallation fractions derived from two chemically different crude oils were also examined. The volume of each distallation fraction administered corresponded to its relative abundance in the crude oil from which it was derived. The inhibitory effect was not associated exclusively with the same distallation fractions from each oil. A highly naphthenic crude oil from the San Joaquin Valley, California, showed the greatest inhibitory activity in the least abundant (2%), low boiling point (smaller than 245 degrees C) fraction and the least inhibitory activity in the highest boiling point (greater than 482 degrees C) most abundant (47%) fraction. In contrast, a highly paraffinic crude oil from Paradox Basin, Utah, showed the greatest inhibitory effect with the highest boiling point fraction and a minimal effect with the lowest boiling point fraction; the relative abundances of these two fractions in the crude oil represented 27 and 28% respectively. Water-soluble extracts of both crude oils also had inhibitory effects on mucosal transfer rates and these roughly proportionate to the inhibitory potency of the low boiling point fraction of each oil. Weathered samples of San Joaquin Valley, California, and the Paradox Basin, Utah, oils showed greater effects than corresponding samples of unweathered oils even though most of the low molecular weight material from both oils was either evaporated or solubilized in the underlying water during the 36-h weathering period.