Luteinizing hormone (LH) depleted ovarian ascorbic acid of immature or adult rats pretreated with gonadotrophins, the former animals being more sensitive than the latter. Follicle-stimulating hormone, luteotrophin and adrenocorticotrophin had minimal or o activity in this assay, whereas vasopressin but not oxytocin had appreciable activity. Vasopressin was more active in the adult rats. If the doses were expressed on a weight basis, vasopressin was actually more potent than the LH standard in the adults; however, endogenous vasopressin release did not deplete ovarian ascorbic acid. The activity of both LH and vasopressin was either not affected or affected to the same degree by hypophysectomy. Retrograde injection of vasopressin into the ovarian vein showed that the action of vasopressin was a direct one on the ovary. Vasopressin does not interfere with the assay of LH in body fluids by this technique.