Development of water stress in leaves of mature Quercus rubra L. caused a marked midday depression in photosynthesis (A) and transpiration (E). At external CO2 partial pressures of 100-110 Pa, a constant temperature of 30 C and a constant photosynthetic photon flux density of about 1000 micromol/sq m/s, A was 8 micromol/sq m at low leaf water potentials (-1.5 to -2.0 MPa), whereas it was 20 micromol/sq m/s in non-stressed leaves (-1.0 ,Pa). At lower external CO2 partial pressures, the effect of low leaf water potential on A was less. The midday depression in gas exchange was relieved by an overnight rain of 2.5 cm. No difference in carboxylation efficiency or CO2 compensation point was found between leaves before and after rain. The relationship between A and E was linear for a given external CO2 partial pressure, but the slope varied with CO2 concentration. Modification of the model of stomatal response proposed by Ball et al. (1987) produced a linear relationship between leaf conductance and a factor incorporating A, relative humidity, and CO2. (Copyright (c) 1990 Heron Publishing - Victoria, Canada.)