Abstract.An empirical model of stomatal response to environmental factors was developed from measurements of stomatal conductance (gs) made in a leaf chamber under controlled conditions. Results presented in a companion paper (Warrit, Landsberg&Thorpe, 1980) indicated that the model could be written in terms of only two factors, photon flux density (Qp) and leaf to air vapour pressure gradient (D). The response ofQpwas hyperbolic and that toDlinear; combining these the equation of the model iswheregris a reference conductance, α is the slope of the response toDand β indicates the sensitivity ofgsresponse toQp. Values of α were 0.20 and 0.30 kPa−1in June and August; the corresponding values of β were 59 and 79 μmol m−2s−1.The model was tested against mean values ofgsobtained with a porometer in the field, using environmental measurements as inputs. Correspondence between measured and calculated values was good. Transpiration rates were calculated from the Penman‐Monteith equation, with stomatal resistance values calculated from the model, and compared with gravimetric measurements of tree water use. It was shown that transpiration could be calculated with acceptable accuracy. The effects of variations in stomatal resistance on transpiration rates under a range of conditions were explored using the model and the Penman‐ Mon