The after-effect of a period of water stress upon the light-opening ability of stomata was studied in tobacco (Nicotiana tabaccum) and broad bean (Vicia faba). Stomatal opening was always measured after floating leaf discs on water under favourable conditions of light and temperature to obtain maximal opening at full leaf turgor. Thus possible interference due to persistence of leaf-water deficits in the post-stress leaves was eliminated. Stomatal aperture was accurately estimated using a resistance porometer; values so obtained were substantiated by the direct measurement of aperture. Two to 4 days of wilting of tobacco, producing leaf-water deficits of 30 to 40 per cent, had a marked after-effect on tobacco stomata. The ability to open in light was depressed and complete recovery from this depression required 2 to 5 days after rewatering. In some cases over-recovery was observed; this was probably related to a physiologically younger condition of leaves of stressed plants following turgor recovery. In beans similar stresses caused after-effects smaller in magnitude but qualitatively similar to those in tobacco. In both tobacco and beans the magnitude of the after-effect was approximately proportional to the leaf-water deficit attained immediately prior to rewatering.