Increases in the net rate of photosynthesis and stomatal conductance of leaves are perceived to occur simultaneously. Published data are re-analysed here to show that when increases in the net rate of photosynthesis are a result of transient increases in the ambient CO₂ concentration then the reverse is the case, i.e. increases in net photosynthesis are closely correlated to reductions in stomatal conductance. Rather than explain this observation in terms of a direct effect of CO₂ on stomata this article develops the idea that stomata respond to a signal in proportion to the degree to which the photosynthetic capacity is realized. This interpretation of stomatal function facilitates the formulation of a simple static model of stomatal responses to the rates of both carbon gain and water loss.