A method is described of measuring dye diffusivity through the cell-wall apoplast of roots, using sections of fresh material cut by hand under paraffin oil. Root pieces of Vicia faba and Glycine max were immersed in a solution of sulphorhodamine G for short periods, and the advance of the dye through the cell-wall apoplast was measured, either in a number of sections cut after various periods of diffusion, or by photographing the same section at intervals while diffusion into it from the surface was observed under the microscope. Fick's second law was used to estimate the diffusivity. Diffusivities, expressed relative to the diffusivity in water (Daq) were in the range Daq/16 to Daq/55 for Vicia roots. The values for Glycine roots showed a wide scatter, with a modal value of Daq/1700. Comparison of this rapid method with the much more laborious stabilization of the dye by freeze-substitution confirmed the values of measured diffusivities in Vicia.