Microdialysis was applied to determine the in vivo transdermal absorption of methotrexate (MTX) in rats with or without a new penetration enhancer, 1-[2-(decylthio)ethyl]azacyclopentan-2-one (HPE-101). A solution composed of 2.5 mM MTX and 3% (w/v) HPE-101 was applied to the shaved abdomen, in which a semipermeable membrane cannula of 10-mm length was inserted intracutaneously with the use of an L-shaped needle. Intradermal microdialysis was performed at a flow rate of 1.0 microL/min for 12 hr. The concentration of MTX in the dialysate was measured by fluorescence polarization immunoassay (FPIA). HPE-101 (3%, w/v) significantly increased the dermal MTX concentration from 0.06 +/- 0.04 microM in the control to 56 +/- 26 microM in the dialysate from 8 to 12 hr. HPE-101 at concentrations of 0.75, 1.5, 2.25, and 3% (w/v) enhanced the total recovery of MTX in dermal dialysate from 0 to 10 hr by approximately 5, 18, 42, and 500 times compared with the control, respectively. The microdialysis system is useful for assessing in vivo transdermal drug absorption.