Seedlings of Zea mays L. (John Innes hybrid) were grown with roots divided between two containers such that part of the root system could reduce the water potential of the soil in its immediate vicinity while the rest of the root system was well supplied with water. When compared to plants rooted in two pots of moist soil, drying of part of the root system resulted in partial closure of stomata, even though leaf water potential, turgor and abscisic acid (ABA) content remained unaffected. When leaf pieces were removed from the two groups of plants and incubated under conditions favourable for stomatal opening, stomata of the ‘half-watered’ plants still showed restricted apertures. Incubation in kinetin (10 mmol m−3) or zeatin (100 mmol m−3) reversed the closure of stomata stimulated by soil drying. These results suggest that a continuous supply of cytokinin from roots may be necessary to sustain maximal stomatal opening and an interruption of this supply due to soil drying may act as an indicator of inhibited root activity, resulting in restricted stomatal opening and thereby restricted water use.