Inhibitory activities of abscisic acid and its analogues on the open pollination of barley (Hordeurn vulgare L.) were studied. Activities of all the analogues as well as abscisic acid were higher than that of MJA. Activities of (+) 8'(9')-alkyl(oxy)-ABAs were higher than that of (+)-8'-fluorinated-ABAs. Activities of (-)-8'-alkyl(oxy)-ABAs were lower than those of each (+)-derivatives, however, those of (-)-8'-fluorinated and 9'-methoxy-ABAs were similar with those of each (+)-derivatives. Based on these resutls, structure requirement for higher activity are discussed. In the spraying experiment of ABA on a field condition, slight inhibition of open pollination were observed.