The growth rate of maize leaves has been investigated for plants grown in pots in controlled conditions and subjected to slow drying over a period of days. The elongation of leaves experiencing water deficit was inhibited primaily during the afternoon. No decrease in the turgor of the growing cells could be detected at that time. Solute concentration in the growing cells increased in tissue experiencing water deficit, but this was shown to occur after the growth rate had fallen. Calculations of the rate of solute accumulation necessary to maintain these concentrations indicated that the rate was less in slowly growing than in rapidly growing cells. The growing tissue of well-watered leaves excreted protons into the apoplastic space, but this acidification decreased in tissue exposed to water deficit. The pH of the apoplastic space correlated with the growth rate of the tissue. In vitro acidification of isolated, frozen-thawed tissue, maintained under constant tension, increased wall extensibility. The results suggest that one role of proton excretion may be to promote wall-loosening events necessary for cell enlargement, and that inhibition of this process may have reduced growth rate in leaves exposed to water deficit.