Abstract:During the development of peanut (Arachis hypogaea L. ) seed, the endogenous ABA content increased steadily‘in hypocotyl, increased to a peak at 40 d after pegging with a drastic decline afterwards in testa; and in cotyledon, increased to a peak at 60 d after pegging but with a slight fall afterwards. There seemed to be a close relationship between the increose of vigor index and net loss of endogenous ABA content in the peanut seed germinating in vitro. Osmoticum (mannitol) promoted the endogenous ABA in the cotyledon and hypocotyl. and Fluridone inhibited that in the cotyledon. There were two different paths of the endogenous ABA synthesis in peanut seed, C40 in the cotyledon and C15 in the hypocotyl. When peanut seeds were put in the conditions of precocious maturation or germination tine endogenous ABA content fell down. Result from this experiment concluded that the hypocotyls played an important role in the transition from development to germination of. peanut seed.