A new method of measuring water uptake by roots is applied to whole root systems of large maize plants growing in aeroponic (mist) culture. The method depends on the build-up of concentration of dye (sulphorhodamine G) on and in the root surface, when it is fed in the mist. Water enters the roots rapidly, but the dye is separated from the water by osmotic filtration and penetrates the apoplast only by very slow diffusion. The dye accumulates progressively with time on roots on transpiring plants, but not on roots of non-transpiring controls. The rate of accumulation of dye at a place on the root is translated to a flux of water into the root at that place, using the concentration of dye in the mist. Water fluxes were measured into first-order branches and axes over the root system and expressed both as fluxes per unit length and per unit surface of root. Values are consistent with those found by potometric methods for limited samples of young plants. The variance of the measurements is quite large, possibly reflecting real heterogeneities in water uptake throughout the root system. The maximum water uptake achieved by a few branches was 40 μl h−1 cm−2 root. On average, flux into axes and branches was the same throughout the root system, at about 5 μl h−1 cm−2. The region of the axis at which the late metaxylem vessels mature and become conducting coincides with the region where the branches become active in water uptake (20–30 cm from the tip). Proximal to this, the branches collect about eight times as much water as the axis, having about eight times the surface area. The zone of maximum water collection by branches and axis is 30–60 cm from the tip (6–8 μl h−1 cm−1 axis). In the older, more proximal regions, water collection drops to about a quarter of this.