The effects of topically applied soluble glucocorticosteroids (methyl-prednisolone and cortisol) were investigated by observation through the microscope to determine their effect on the canine basilar artery and the canine and monkey arterial circle of Willis made spastic by puncture or by topical barium chloride, prostaglandin (F-2alpha), or serotonin. The steroids were tested without their normal diluent; the diluent was tested separately. Marked vasodilatory effects of the major arteries and the smallest visible arteries were found with both of these steroids. Moderate vasodilatory effects also were found with the diluent alone. In addition methylprednisolone was injected into the vertebral artery (40 mg) and intravenously (10 mg/Kgm); vasodilation beyond that seen in the absence of drug injection was not observed. In monkeys whose circle of Willis was made spastic by arterial puncture, severe spasm persisted at 24 and 72 hours. If the puncture was followed by topical application of 10 mg of 4% methylprednisolone acetate, spasm was not seen at 24 or 72 hours.