trans-Aconitate ion, an inhibitor of the tricarboxylic acid cycle, was identified in range grasses as trans-aconitic acid, which was isolated in crystalline form. It occurs in surprisingly high concentrations in early-season forage grasses. Dry-weight concentrations of trans-aconitate vary with season and species; concentrations of between I and 2.5 percent are common in mixed pasture grasses, but are higher in certain species such as Hordeum leporinum (3.5 percent) and Phalaris tuberosa var. stenoptera (4.2 percent). Leaves of western larkspur (Delphinium hesperium) contain 12.2 percent trans-aconitate. trans-Aconitate may be partially responsible for nutritional disorders, such as grass tetany (hypomagnesemia), that occur in grazing cattle in early spring.