The evapotranspiration component, based on the Penman-Monteith equation, of the distributed Système Hydrologique Européen (SHE) model of catchment hydrology is briefly described. The importance of evapotranspiration predictions to the operation of the model is stressed. However, such predictions may be expected to be in error and ideally it would be desirable to predict the error variance associated with estimates of evapotranspiration. This is not yet possible but the sensitivity of such estimates to errors in input data and estimated model parameter values can be investigated. It is shown that for dry canopy conditions at three sites within a broadly humid temperate region, the sensitivity of Penman-Monteith estimates of evapotranspiration to different input data and parameters is very dependent on the values of the aerodynamic and canopy resistance parameters that introduce the influence of vegetation type into the predictions. For forest surfaces in particular, the evapotranspiration predictions are highly sensitive to values of the canopy resistance, so that accurate estimation of this parameter is important.