Gas exchange and leaf water potential were investigated with regard to water stress in potted 6-month old plants of Copaifera langsdorffii. Leaf water potential (Ψ = -2.0 MPa) in stressed plants was near the osmotic potential (π = -2.2 MPa) at 0900 h during 22 d with water stress. Transpiration rates (if were in linear relation to stomatal conductance (gs) during stress, while water potential and net photosynthetic rate (PN) at 0900 h were in a curvilinear relation. More pronounced decrease of P N occurred only at gs between 0.01 to 0.08 mol nr2 s-1. Increase in water use efficiency (WUE) coincided with morning Ψ diminution from -2.0 MPa to -2.2 MPa. This 10 % decrease in Ψ corresponded to a 60 % decrease in gs (from 0.15 to 0.06 mol m*2 s*1). When pre-dawn Ψ (-3.4 MPa) was close to the Ψ measured at 0900 h (-3.5 MPa), both the and gs were around zero. Therefore, Ψ at 0900 h, π and values of gas exchange were closely related during the days of stress. Values of Ψ for zero and for stomatal closure (-3.5 MPa) were in the same range for sclerophyllous plants.