The purpose of this study was to compare the intrinsic absorptivity of piperacillin in the jejunum and the nasal cavity, to investigate the effect of bile salts, fatty acids and their mixed micelles on the intestinal and nasal absorption of piperacilIin, to examine the reversibiIity of bile salt-fatty acid mixed micelles absorption promoting action and to design an effective intranasal drug delivery system for antibiotics. And absorption promoters used were bile salts [sodium cholate (NaC), sodium glycocholate (NaGC)], unsaturated fatty acids [oleic acid (OA), linoleic acid (LA)] and their mixed micelles (NaC-LA). The present study employed the in situ nasal and intestinal perfusion technique in rats. The apparent permeabilities $(P_{app})$ of piperacillin were $0.40{pm}0.04{ imes}10^{-5}cm/sec(mean{pm}S.E)$ in the jejunum and $1.32{pm}0.08{ imes}10^{-5};cm/sec$ in the nasal cavity, which indicated that intrinsic absorptivity of piperacillin was greater in the nasal cavity than in the jejunum. When absorption promoters were used in the rat nasal cavity, the decreasing order of apparent piperacillin permeability $(P_{app},;10^{-5};cm/sec)$, corrected for surface area of absorption, was NaC-LA $(4.62{pm}0.16)$> NaC $(4.36{pm}0.32)$>LA$(2.24{pm}0.26)$ NaGC $(2.17{pm}0.21)$>OA $(1.53{pm}0.16)$. The increase in permeability of piperacillin was 3.5-fold in the rat nasal cavity and 1.5-fold in the rat jejunum for formulations containing NaC-LA mixed micelles as compared to those without absorption enhancer. The effect of NaC-LA mixed micellar solutions was synergistic and was greater than that with single adjuvant. The reversibility of nasal mucosal permeability was observed within approximately 2 hr after removal of NaCLA mixed micelles from the nasal cavity. These results suggest that NaC-LA mixed micelles can be used as nasal mucosal absorption promoters of poorly absorbed drugs.