Two different fractions of manganese were found in the maize plant root apoplasm (intercellular space containing cell walls) after soaking the roots in MnCl2 solutions (concentration range 0.01-10 mmol.l-1): (a) an Mn2+ fraction in the water free space (WFS) which gave a characteristic six-line spectrum, and (b) an immobilized fraction that gave no detectable ESR spectrum. Both fractions affect proton NMR relaxation (T1) of the tissue water through water exchange across cell membranes. ESR spectra of free and total manganese of the root tissue treated with MnCl2 also revealed different time courses for saturation of WFS and DFS with Mn2+. Binding of manganese in the extracellular space of the tissue seems to be the rate limiting step in permeation of Mn2+ across the root cell membranes.