Net photosynthesis, transpiration, and stomatal conductance of terminal shoots of Douglas-fir (Pseudotsuga menziesii(Mirb.) Franco) were measured using an open gas exchange system. Correlations between these physiological parameters and environmental variables on an overcast day, a cool partly sunny day, and a day of high temperature and leaf–air vapor pressure difference are presented. Diurnal variation in shoot water potential and intercellular space CO2concentration had little effect on the physiological parameters. Leaf–air vapor pressure difference and (or) leaf temperature had considerable influence on days of high temperatures. Net photosynthetic rate was strongly correlated with photon flux density on completely overcast days.Stomatal conductance exerted little control on diurnal variation of net photosynthetic rates on overcast days because stomatal conductance saturated at lower photon flux densities than net photosynthesis. When net photosynthesis was light saturated parallel responses of stomatal and residual conductances to vapor pressure difference and (or) leaf temperature occurred. Intercellular space CO2concentration remained fairly constant when net photosynthesis was light saturated.