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THE plant hormone abscisic acid (ABA) is thought to be involved in the regulation of transpiration through its effects on stomatal aperture1. Application of synthetic ABA causes stomatal closure in whole plants2, shoots3 and leaf epidermal strips4. During water stress, levels of the hormone increase in plant tissues5, including the leaf epidermis6, and this is usually associated with stomatal closure7. There is evidence that ABA affects the ionic8,9 and metabolic9 status of the stomatal complex and it has been concluded that ABA acts directly on guard cells in effecting stomatal closure8,10,11. There is, however, little information about the distribution of ABA in specific cells of the epidermis or about the sensitivity of the stomatal complex to ABA. We present here evidence that ABA can accumulate in the stomatal complex of Commelina communis L., a species used extensively in studies of stomatal physiology12. The leaves of this plant yield relatively uncontaminated epidermal strips13 and this property has made it possible to calculate the apparent sensitivity of the stomatal complex to ABA. The evidence is based on experiments involving the uptake, distribution and metabolism of exogenous 2-14C-ABA.