The oral bioavailability of such ionized drugs as ceftibuten, salicyllc acid, and famotidine, when coadministered with enteral nutrition support was assessed in rats. Orally active nutrition support (HepanED^<[O!R]>, Twinline^<[O!R]>. Enterued^<[O!R]>) has exhibited an inhibitory effect on the absorption of ceftibuten from the rat intestine, thus suggesting this drug to not demonstrate the desired therapeutic effect. In Loop experiments using the rat intestine, the absorption rates of ceftibuten, salicylic acid and cimetidine were found to decreas to similar extents when combined with enteral feeding products. The suppression by such enteral feeding products is considered to be due to the interaction between the drug and the component(s) of the oral nutrition support, but is not thought to be related to the gastric emptying time. On the other hand, the absorption of levofloxacin, a zwitterlonlc compound, from the intestinal loop was not affected at all by HepanED^<[O!R]> This result correlates with the previous data^< 4)> which showed no inhibitory effect of Enterued^<[O!R]> on cephalexln absorption. These findings therefore suggest a common relationship between the structures of the drug molecules and the inhibition behavior observed during enteral nutrition support.