The effects of prostaglandin E1, PGE, and of epinephrine-(Epi) on the dynamics of insulin release from rat islet, in vitro, was investigated. PGE1, 0.3 ¼g/ml produced biphasic insulin release in the presence of 50 mg/100 ml glucose, but inhibited both phases of insulin release induced by 300 mg/100 ml glucose. Further, Epi produced a dose dependent inhibition of both phases of glucose induced insulin release. Addition of PGE1 reversed this effect of Epi such that insulin release in the presence of high concentrations of Epi plus PGEj plus glucose were greater than in the presence of glucose alone. (Endocrinology94: 835, 1974)