Stomata opening from darkness in constant light and in air of the normal carbon dioxide content unsaturated with water vapour come to different steady-state apertures at different light intensities. This shows that one or more negative feedback loops are operating. With increasing stomatal apertures, three feedback processes must depend respectively on (1) increased entry of carbon dioxide, (2) increased loss of water vapour, and (3) the consequent lowering of the leaf temperature; the effect of the first of these on the stomata must be reduced by a fourth loop in which some of the extra carbon dioxide is taken up by mesophyll photosynthesis. Experiments were performed in which these four effects were thought to be eliminated. In Tradescantia virginiana there was then no evidence of a difference in steady-state aperture at two light intensities that differed by an order of magnitude; in Commelina communis, however, the steady-state apertures differed significantly, implying that there were still important negative feedback processes operating -- perhaps connected with active uptake and passive loss of ions by the guard cells. These results appear to have an important bearing on the calculation of feedback loop gains. The question is discussed: why, with negative feedback apparently eliminated, do T. virginiana stomata not continue to open indefinitely but come to a steady-state aperture?