The cell water balance of maize ( Zea mays L., cv LG 11) and pearl millet ( Pennisetum americanum L., cv MH 179) during osmotic stress was studied non-invasively using 1 H nuclear magnetic resonance (NMR) microscopy. Single NMR parameter images of (i) the water content (ii) the transverse relaxation time ( T 2 ) and (iii) the apparent diffusion coefficient ( D app ) were used to follow the water status of the stem apical region during osmotic stress. During stress there are hardly any changes in water content or T 2 of the stem region of maize. In contrast, the apical tissue of pearl millet showed a ~ 30% decrease of T 2 within 48 h of stress, whereas the water content and D app did not change. These changes can be explained by an increase of the membrane permeability for water. This conclusion is supported by results from scanning electron microscopy, relaxation measurements of sugar solutions and numerical simulations of the relaxation and (apparent) diffusion behaviour of water in a plant cell.