Ethacrynic acid is a well known diuretic drug. We present here evidence that this compound is also an antimicrotubular agent which inhibits the brain tubulin polymerization and can displace 3H colchicine from its binding site. Since 2-mercaptoethanol reversibly inhibits the anti-microtubular properties, we conclude that ethacrynic acid binds covalently to brain tubulin through a cysteine near the colchicine site.