The in vitro clearances of the 5HT3 antagonists, ondansetron, alosetron and GR87442 were investigated. Intrinsic clearances using either metabolite formation or substrate depletion methods were equivalent (R2 = 0.95). Hepatocytes from preclinical species were superior to microsomes for the prediction of hepatic clearance (CL(H)), whereas the predictions from human microsomes and hepatocytes were similar. Using a non-restrictive model, seven of the nine CL(H) predictions using hepatocytes were within 2-fold of the in vivo CL(H) values. If the unbound fraction was included, the clearance of the compounds was generally under-predicted by both in vitro models. However, for the most metabolically stable compound, GR87442, the non-restrictive model over-predicted CLp. This and the possibility of extrahepatic metabolism indicate that the restrictive model is more appropriate for prediction of CL(H). The rank order of metabolic stability correlated with that in vivo. All three compounds were more metabolically stable in human than in the preclinical animal species examined.