The threshold leaf water potential required to initiate stomatal closure in cotton (Stoneville 213) became progressively more negative when plants were subjected to a series of water stress cycles. The shift in the threshold water potential required for induction of stomatal closure was dependent on the number of previous stress cycles and leaf age. The basal level of endogenous abscisic acid (ABA) in fully turgid leaves increased in response to the stress treatments, whereas the amount accumulated in response to a subsequent stress did not differ greatly among plants that had experienced different degrees of stress conditioning.Stomatal sensitivity to (+/-)-ABA fed through the transpiration stream was enhanced in detached leaves of plants which had experienced repetitive water stresses. The increased sensitivity was apparently the result of ABA synthesized during the stress periods since foliar applications of ABA sensitized stomata in an analogous manner. Furthermore, the amount of (+)-ABA required to initiate stomatal closure in leaves from the various stress treatments was not related to the amounts accumulated during wilting.