5-(2-Hydroxy-1-phenyl (or-1-naphthalenyl)ethylidene) and 5-(2-oxo-1-phenyl (or 1-naphthalenyl)ethylidene)-2,4-thiazolidinediones and derivatives thereof are useful in lowering blood glucose levels in a hyperglycemic laboratory animal model and are thus useful in the treatment of diabetes mellitus.