X-ray microangiography was used to investigate the role of basal EDRF activity in the isolated rabbit ear, changes in perfusion pressure at different flow rates being correlated with simultaneous changes in diameter in resistance arteries 70–1,000 µm in size. Under conditions of controlled-pressure but not controlled-fiow perfusion the preparations were shown to autoregulate flow, but only when EDRF activity was inhibited by haemoglobin or L-NMMA. The diameter data indicated that this phenomenon was mediated by a flow- and/or pressure-dependent constrictor response that is normally suppressed by EDRF activity. We also investigated the influence of basal EDRF activity on the geometrical ‘optimality’ of resistance artery branching, using four models which minimise respectively the total surface area, volume, shear stress (drag) or power losses at bifurcations. EDRF activity was found to maintain optimality in terms of minimum volume and power losses over a wide range of flow rates in pharmacologically constricted preparations. This may allow rapid changes in flow to occur with only small changes in central arterial pressure and also help to minimise cardiac work.