The present investigation attempts to increase intestinal permeability and hence absorption of biopharmaceutic classification system (BCS) Class III (cefotaxime sodium (CX)) and Class IV (cyclosporin A (CSA)) drugs by employing certain absorption enhancers. Drugs were co-perfused with sodium caprate (SC, 0.25% w/v), piperine (P, 0.004% w/v) and sodium deoxycholate (SD, 1.0% w/v) separately in rat in situ single pass intestinal perfusion model. These additives increased intestinal permeability (P(app)) and absorption rate constant (K(a)) up to two and fourfold, respectively. SC exhibited substantial absorption enhancement of both CX and CSA, while SD and P enhanced absorption of CX and CSA, respectively. Co-administration of SC significantly enhanced peroral bioavailability of CX (from 29.4 +/- 1.7 to 69.6 +/- 3.2) and CSA (from 18.4 +/- 15.6 to 49.6 +/- 25.1) in rats, while P increased bioavailability of CSA (from 18.4 +/- 15.6 to 33.1 +/- 17.7). Transmission electron microscopy of intestinal mucosa revealed that SC and SD act on lipid and protein domains of absorptive membrane. P showed no effect on intestinal P(app) and oral bioavailability of CX but has a profound effect on CSA, a known P-glycoprotein (P-gp) substrate. These results indicated that P enhances intestinal absorption of CSA by modulating P-gp mediated efflux transport. Release of lactate dehydrogenase in situ from intestinal mucosa in the presence of absorption enhancer was taken as index of its local toxicity. All the absorption enhancers showed significantly less release of LDH compared to positive control, sodium dodecyl sulfate (60% w/v). Overall, the data indicate that the features of these commonly used food ingredients or endogenous bile salts can effectively improve bioavailability of various BCS Class III and Class IV drugs.