A model was developed to describe stomatal conductance as a function of photosynthetically active radiation, leaf water potential, leaf temperature, vapor pressure difference (leaf interior minus leaf surface) in Acer saccharum Marsh. The model used a multiplicative approach and used a new method for determining the functional dependence of stomatal conductance on individual variables. The functions: (1) were expressed using a linear spline technique; (2) were constrained by very general mechanisms of stomatal action rather than specific analytical equations; and (3) were directly determined from data measured in the field. The spline functions were fit by least square estimation applied to data under uncontrolled field conditions. This is in contrast to the normal method of measuring functions from experiments conducted under controlled conditions. The data used for spline fitting were obtained in the field from more than 40 detached branches on about 30 days during various weather conditions throughout two summers (1990 and 1992). Our model predicted nearly 80% of the observed variation in leaf stomatal conductance and 90% of the variation in transpiration for leaves on mature trees and on excised branches when leaf water potential was q -2 MPa.