Pioglitazone (5-(4-(2-(5-ethyl-2-pyridyl)ethoxy)benzyl)-2,4-thiazolidinedione, 2) is a prototypical antidiabetic thiazolidinedione that had been evaluated for possible clinical development. Metabolites 6-9 have been identified after dosing of rats and dogs. Ketone 10 has not yet been identified as a metabolite but has been added to the list as a putative metabolite by analogy to alcohol 6 and ketone 7. We have developed improved syntheses of pioglitazone (2) metabolites 6-9 and the putative metabolite ketone 10. These entities have been compared in the KKAy mouse model of human type-II diabetes to pioglitazone (2). Ketone 10 has proven to be the most potent of these thiazolidinedinediones in this in vivo assay. When 6-10 were compared in vitro in the 3T3-L1 cell line to 2, for their ability to augment insulin-stimulated lipogenesis, 10 was again the most potent compound with 6, 7 and 9 roughly equivalent to 2. These data suggest that metabolites 6, 7 and 9 are likely to contribute to the pharmacological activity of pioglitazone (2), as had been previously reported for ciglitazone (1).