The metabolites of (±)-5-[p-[2-(5-ethyl-2-pyridyl)ethoxy]benzyl]-2, 4-thiazolidinedione (1, pioglitazone), which is a representative insulin-sensitizing agent, were synthesized to confirm their structures and for studies of their pharmacological properties. Of the metabolites identified, a compound hydroxylated at the 2-position of the ethoxy chain (3) and compounds oxygenated at the ethyl side chain attached to the pyridine ring (4, 5) were found to be active, although the potency was slightly lower than that of the parent compound.