Suboptimal N nutrition and leaf aging both increase stomatal closure in water-stressed cotton (Gossypium hirsutum L.) plants or in excised leaves supplied abscisic acid (ABA). The enhanced response to ABA could be from altered partitioning of ABA between inactive (symplastic) and active (apoplastic) pools, or from altered responsiveness of guard cells to ABA. Excised leaves were preloaded with (14)C-ABA, and the apoplastic pool of ABA was isolated by pressure-induced sap exudation in a pressure chamber. ABA content of the exudate was increased greatly by dehydration to zero turgor, and was increased somewhat less by N deficiency or leaf aging (at positive turgor). The increases in apoplastic ABA were sufficient to initiate stomatal closure. Kinetin (supplied with the ABA) did not alter partitioning regardless of leaf N status or age. Likewise, kinetin did not alter stomatal sensitization to ABA dim to aging. However, it did block sensitization due to N deficiency. Distribution of ABA between pools suffices to explain the effects of leaf age on stomatal behavior, but in the case of N deficiency there are additional kinetin-reversible effects, presumably at the guard cells.