An early symptom of N or P deficiency in cotton seedlings is inhibition of leaf expansion. This inhibition is mostly confined to the daytime, when plants transpire rapidly. Evidence indicates that the nutrient stresses decrease root hydraulic conductivity (efficiency of water transport), thereby increasing the water deficit in the tops during the daytime when large amounts of water are transported through the roots. Using a pressure probe, we examined the water relations of individual cells in the root cortex. Nutrient stress (growth on -N or -P nutrient solution) quickly increased cellular turgor and the half-time for water exchange. N stress also increased the modulus of elasticity. Calculated hydraulic conductivity decreased continuously with increasing plant age in the controls, but decreased much faster in the nutrient stressed plants. The cellular hydraulic conductivity was closely correlated (r = 0.99) with cotyledon expansion rates across all nutrient treatments. Root hydraulic conductivity (determined earlier by different methods) was 30 to 70% of cell hydraulic conductivity, implying that most of the water flows through the root via an apoplastic pathway. The data indicate that in roots of nutrient-stressed plants, biophysical properties of cell membranes, probably in the endodermis, limit the rate of water passage to the stele.