In this study we determined the time-course for the onset and the loss of the effect of short-term feeding rats isocaloric semisynthetic diets containing a high content of saturated (HS) or polyunsaturated (HP) fatty acids on the jejunal and ileal uptake of medium- and long chain fatty acids, cholesterol and glucose. Animals were fed HP or HS for 3, 7 or 14 days; then the diet was switched to standard Purina rat chow for a further 3, 7 or 14 days. The uptake of medium chain fatty acids was unchanged. The differences between HP and HS in glucose uptake occurred within 3 days, but persisted for 14 days, whereas there were qualitative as well as quantitative changes in the pattern of lipid uptake: differences in uptake of stearic, oleic, linoleic and linolenic acids and cholesterol occurred after 7 days of feeding HP or HS. Jejunal uptake of linoleic acid was greater in HP than HS on day 7, but HS was greater than HP on day 14. The effect of diet on lipid uptake was similar in the jejunum and ileum. The altered uptake of stearic and oleic acids persisted after the rats were switched back to chow, whereas the uptake of the other nutrients became similar. Thus, changes in dietary content of saturated and polyunsaturated fatty acids have early effects on intestinal transport function; some of these changes persist even when animals are returned to feeding on chow; and glucose transport is rapidly altered by dietary changes, whereas lipid uptake changes only after 7 days.(ABSTRACT TRUNCATED AT 250 WORDS)