We outline a theoretical framework for comparing the relative effectiveness of hormonal and other factors in the control of physiological responses. This involves determining a 'baseline' set of conditions, then adjusting each factor in turn with the others held constant. The initial rate of response to each change in condition is estimated and compared among factors to arrive at a quantitative indication of control potential under the specified baseline conditions - the control coefficient. The suitability of the stomatal response as a model system for such studies is discussed. A portable infra-red gas analyser was used to investigate the control potential of abscisic acid (ABA), CO2 and light - measured as photosynthetic photon flux density (PPFD) - over responses of Phaseolus vulgaris L. stomata at baseline conditions appropriate to an unstressed plant under moderate light conditions. The ranking of control coefficients observed under these conditions was light>CO2>ABA. The control coefficients of CO2 and ABA were 0.28 and 0.11 times that of light, respectively. These results are discussed in relation to the sensitivity of the plant material and the experimental conditions. Implications for a hormonal role for ABA in the control of stomatal movements are considered. We conclude that this method can provide valuable information concerning the relative control potential of hormonal and other influences in the context of differences in baseline conditions and in relation to changes in sensitivity of plant material.