An aqueous extract was made from black walnut (Juglans nigra) heartwood obtained in the fall of the year. Ten hours after nasogastric administration of 5 L of the extract, a 550-kg, 13-yr-old Quarter Horse gelding experienced Obel grade-3 laminitis. The effect of aqueous extract of black walnut on vascular contractility was then tested, using isolated equine digital arteries and veins. The vessels were maintained in Krebs bicarbonate buffer with 95% oxygen at 37 C. The extract did not induce a direct contractile effect. It did, however, reversibly enhance the vasoconstriction induced in the isolated vessels by administration of epinephrine potentiated with hydrocortisone. In contrast, aqueous extracts made, using the same techniques, from the shavings of eastern white pine (Pinus strobus), eastern red cedar (Juniperus virginiana), and pin oak (Quercus palustrus) had no effect on epinephrine-induced digital vessel contractions.