Topical application of the 5-HT3 receptor antagonist ondansetron (50-250 microg) delivered in a pluronic lecithin organogel vehicle (PLO, 0.5 ml) produced dose-dependent attenuation of nociceptive and inflammatory effects of intradermally injected capsaicin (10 microg/10 microl) in humans. Significant, dose-dependent analgesic effects were produced by 100 microg and 250 microg doses of ondansetron; these doses also reduced mechanical hyperalgesia produced by capsaicin. However, only 250 microg dose of ondansetron diminished capsaicin-induced inflammatory flare.