The effect of capsaicin on intestinal cefazolin absorption was examined by means of an in situ closed loop method in rats to clarify whether the vanilloid receptor (TRPV1) is involved in drug absorption driven by passive diffusion. In control experiments with 1 mg/mL cefazolin, the amount of cefazolin absorbed from the closed loop was 15.3+/-1.5 microg/cm in the rat jejunum. The absorption amount was increased to 22.8+/-0.9 and 23.4+/-2.4 microg/cm when capsaicin was applied with cefazolin at concentrations of 10 and 400 microM, respectively. The enhancing effect of capsaicin on cefazolin absorption was suppressed when ruthenium red, a non-selective inhibitor of transient receptor potential (TRP) cation channels, was intravenously infused into the rat during the experiment. Cefazolin accumulation in the intestinal tissue was not altered in the presence of capsaicin. Collectively, the mechanism accounting for the capsaicin-induced increase in the intestinal cefazolin absorption is probably that capsaicin associating with TRPV1 increases the intrinsic permeability of cefazolin in intestine.