We estimated the reduction of transpiration from drought for tree species in a mixed boreal 60-year-old stand in central Sweden. Actual transpiration was estimated from direct measurements of sap flow rate in Pinus sylvestris and Picea abies trees during two consecutive years with contrasting precipitation. Drought-induced reduction of transpiration (transpiration deficit) was quantified as the difference between the measured sap flow and the transpiration calculated for non-limiting soil water conditions. The drought-free transpiration was estimated on an hourly basis from Penman-Monteith equation with the parameterized canopy conductance (g(c)) functions for individual species. The values of g(c) for fitting a two-parameter function of radiation and vapour pressure deficit were obtained for a 3-d period by inverting the Penman-Monteith equation. Canopy conductance of pine was similar relative to spruce on ground area basis. This made g(c) of pine larger relative to spruce per leaf area unit, since pine tree foliage mass was about one third that of spruce. Transpiration deficit was small in the growth season of 1995. It reached about 10 % for spruce during the summer months. In 1994, however, the transpiration deficit was large for both species and extended throughout most of the growth season. During summer 1994, the decreased canopy conductance caused a 20 and 22 % reduction in gross photosynthesis for pine and spruce, respectively, indicating a loss of production of at least that proportion. Pines were less sensitive to drought spells as compared to the more shallow-rooted spruces. On the other hand, spruce utilised the precipitation incoming in small quantities more effectively and responded faster. Species composition of boreal forest can affect stand scale fluxes and this should be recognised by process models. ((C) Inra/Elsevier, Paris.).