Secretin was isolated in the form of a crystalline picrolonate. 0.08 mgm. of this compound constituted an Ivy-dog unit, and 0.004 mgm. a Hammarsten-cat unit of secretin. 0.014 mgms. of the free secretin prepared from the picrolonate constituted an Ivy-dog unit. The crystalline picrolonate melted sharply with decomposition at 234-235[degree] C. Picrolonic acid made up most of the molecule, which possessed the empirical formula C3H3ON.