The chemistry of vanadium compounds that can be taken orally is very timely since a vanadium(IV) compound, KP-102, is currently in clinical trials in humans, and the fact that human studies with inorganic salts have recently been reported. VO(acac)2 and VO(Et-acac)2 (where acac is acetylacetonato and Et-acac is 3-ethyl-2,4-pentanedionato) have long-term in vivo insulin mimetic effects in streptozotocin induced diabetic Wistar rats. Structural characterization of VO(acac)2 and two derivatives, VO(Me-acac)2 and VO(Et-acac)2, in the solid state and solution have begun to delineate the size limits of the insulin-like active species. Oral ammonium dipicolinatooxovanadium(V) is a clinically useful hypoglycemic agent in cats with naturally occurring diabetes mellitus. This compound is particularly interesting since it represents the first time that a well-characterized organic vanadium compound with the vanadium in oxidation state five has been found to be an orally effective hypoglycemic agent in animals.