Two permanent sampling sites were selected at the two extremes of a rainfall gradient in natural forests of Quercus pyrenaica Willd. located in the Sierra de Gata (Salamanca Province, Spain). During 1991 and 1992, diurnal courses of transpiration rate (E), stomatal conductance (g(s)) and leaf water potential (Psi(l)) were studied at different levels in the tree canopy. Leaf water potential fell from a high value in the early morning to a minimum at midday and then increased again in the afternoon. Values of Psi(l) in the early morning and at midday were lower at Fuenteguinaldo (dry site) than at Navasfrías (wet site) and were related to soil water availability. Stomatal conductance increased during the morning and maximum g(s) occurred about 3 h before Psi(l) was at its lowest value. The highest values of g(s) were found at Navasfrías during July. The lowest Psi(l) values were found in the upper parts of the canopy and differences among canopy levels were greatest when soil water availability was low. In August, minimum values of Psi(l) differed among canopy levels by 1.0 MPa at Navasfrías and by 0.8 MPa at Fuenteguinaldo. The maximum difference in g(s) among canopy levels was about 150 mmol m(-2) s(-1) at Navasfrías and about 300 mmol m(-2) s(-1) at Fuenteguinaldo. The results indicate a nonconservative pattern of water use in Quercus pyrenaica.