The effect of drought on the photosynthetic functioning of two C3 plants, Phaseolus vulgaris and Elatostema repens, has been examined. Leaf net CO2 uptake measured in normal air was negligible at a leaf water deficit of about 30% while the calculated leaf intercellular CO2 concentration (Ci) was unchanged. However, both the maximal photosynthetic capacity (CO2-dependent O2 evolution) and apparent quantum yield, measured in the presence of saturating CO2 levels (5 to 14%), only started to decrease within the range of 25 to 30% leaf water deficit. This shows that the drought-induced inhibition seen in normal air is not caused by an inhibition of the photosynthetic mechanism, and that in this case Ci values can be misleading. Both 77 K and room-temperature fluorescence measurements indicate that the functioning of the photosystem-II reaction centre is hardly modified by water shortage. Furthermore, an analysis of photochemical chlorophyll fluorescence quenching shows, in the absence of CO2, that O2 can be an efficient acceptor of photosynthetic energy, even in severly dehydrated plants which do not show net CO2 uptake in normal air. In these plants, O2 is probably reduced mainly via Mehler-type reactions. High-light treatment given at low O2 increases photoinhibition as measured by the decrease of apparent quantum yield in dehydrated P. vulgaris, whereas, interestingly, 1% O2 protects dehydrated E. repens against high-light damage. The two plants could have different protective mechanisms depending upon the O2 level or different photoinhibitory sites or mechanisms.