Root growth was analysed in 6 to 8-day-old seedlings of wheat (Triticum aestivum L.) during exposure to low O2 concentrations by measuring the length of epidermal cells at fixed points along the root axis. The effect of O2 deficiency on cell elongation and cell production was inferred from estimates of cell length and the rate of root extension. Epidermal cells ceased elongating at a point 6 mm from the root tip. The rate of cell elongation reached a maximum between 1 and 2 mm from the apex, and the position of this zone was independent of O2 concentration. The final length of epidermal cells and the total number of cells displaced from the elongating zone per hr (cell flux) were reduced by 20 and 44%, respectively, in roots grown to 0.055 compared with 0.115 and 0.26 mM O2 (aerated solution). In roots exposed to 0.01 mM O2, cell degeneration was evident in apices after 48 hr, presumably due to a lack of ATP required for maintenance of membrane structure and solute transport.