Abstract: Abscisic acid (ABA) participates in the control of diverse physioogical process. Although the molecular mechanism of ABA action is unknown, ABA receptor(s) have not yet been identified, high affinity of ABA binding proteins in some plants, either membrane-bound or soluble, have been described. The sense of ABA lies in the outside or inside of plasma membrane, there are some evidences with the techniques of microinjeetion. Considerable progress has been recently made in the characterization of more downstream elements of the ABA regulatory network, especially in the control of stomatal aperture by ABA. The details of the underlying signalling pathways start to emerge.