The interactive effects of light and drought on water relations and soluble sugars were addressed on Quercus suber L. seedlings grown under the combination of four irradiances and two soil water contents. Leaf water potentials at predawn and midday were lower for water stressed seedlings, independently of light environment. Osmotic potentials at full (Psi pi(100)) and zero turgor (Psi pi(100)) decreased with drought, under the four light treatments. However, the decline was greater with the development of water stress under the two treatments of higher light availability. The higher soluble sugar concentrations in seedlings grown under higher irradiances provoked a decrease of Psi pi(0). The decrease in Psi pi(0) and Psi pi(0) in moderate stressed seedlings was accompanied by an increase in epsilon(max). The lower ability for osmotic adjustment in seedlings grown under moderate water stress and low light environment suggested a lower efficiency in developing physiological mechanisms for drought tolerance in shade- grown seedlings.