Abstract: AIM: The transdermal absorption of ondansetron in rats with or without a widely used enhancer azone was studied. METHODS: A new in vivo sampling technique, cutaneous microdialysis, was used to determine the drug concentration in dermis in rats. And the recovery of the probe was assessed in vivo according to the retrodialysis method. RESULTS: The concentrations of ondansetron in the dialysate and the dermis were increased greatly by co-application of azone. Two percent and five percent (v/v) azone were shown to improve the steady state flux of ondansetron in dialysate from 0.001 μg.h^-1 to 1.281 and 1.824 μg.h^-1, respectively. CONCLUSION: The cutaneous microdialysis is a powerful tool for the study of cutaneous drug penetration in vivo, and azone is an effective enhancer for the cutaneous penetration of ondansetron in rats.