Effects of drought on photosynthesis of two co-occurring oak species (Quercus robur and Q. petraea) have been assessed under natural conditions in a 30-y-old stand. Leaf conductance and net CO2 assimilation rates have been measured at midday in situ during two successive summers (1991 and 1992). Chlorophyll a fluorescence parameters were monitored before dawn during the summer of 1991. A severe drought was imposed on two trees per species, resulting in decreases of pre-dawn leaf potential to about –2.0 MPa during both years. Both species showed the same overall response to drought: gradual decrease of net assimilation and leaf conductance but maintenance at substantial levels down to strong water deficits, stability of the pre-dawn photochemical efficiency of PS II during the whole period of drought, and declines in pre-dawn fluorescence (both basic and maximal levels) owing probably to changes in the optical properties of leaves during maturation. It was concluded that both species displayed a strategy of tolerance to drought, and they displayed efficient protection mechanisms against permanent high irradiance damage. Slight differences were observed between both species, Q. robur displaying smaller leaves, lower chlorophyll contents, and a larger stomatal conductance at equivalent net assimilation rates than Q. petraea. Significance of these findings is discussed in relation to the ecological requirements known for each of these species.