Abstract: In the Mediterranean region, water and carbon balances are important factors tounderstand cork oak forest decline. The objective of this investigation is to know the seasonalvariations of cork oak transpiration and net photosynthesis in the North of Tunisia. Theexperimental design was carried out during the 2009 vegetative season. It consisted in followingupthe sap flow densities (4 trees) and in monthly measurements of photosynthesis rates (6 trees).Usual climatic parameters, such as soil moisture, radiation, temperature, wind velocity andhumidity were measured using an automatic weather station. Tree transpiration (Tr) indicates asimilar evolution to climatic demand (ETo Penman) with a weak slowing down during July-August. Tree transpiration accounts for 40% of ETo and 30% of annual rainfall and has reached amaximum value of 2.8mm/j when ETo is almost 7mm/j. Average photosynthesis marked a cleargrowth from March to June, reaching a maximum value of Pn = 13μmol/m2 s -1, and droppedappreciably during August and July (Pn = 4μmol/m2 s -1).