The energy balance of an upland heath dominated by heather (Calluna vulgaris) was measured in dry and wet weather. Median values of both transpiration and evaporation rates were ca. 2 mm hr-i. The median Bowen ratio for the dry canopy was 2.0 and for the wet canopy 0.6. On dry days the median value of the saturation deficit was only 3.8 mb and that of the climatological resistance was 30 s m-'. The bulk stomata1 resistance increased from ca. 50 s m-' in the morning to over 290 s m-' in the afternoon with an overall median value of 110 s m-i. Transpiration from the dry canopy was controlled by a combination of small saturation deficits and large stomata1 resistances. The median value of the boundary-layer resistance of the canopy was 22 s m -i and was low partly because of a large low-level drag coefficient. Saturation deficits on wet days were close to zero and evaporation of intercepted water proceeded at close to the equilibrium rate, being largely limited by the low fluxes of available energy. The water loss from heather was compared with simulated losses from coniferous forest, herbaceous crops and grassland in the same conditions to evaluate the effects of vegetation on water loss from catchments.