The microsphere technique for studying renal blood flow is based on injection of a small volume containing radioactively labelled microspheres into the left atrium, left ventricle or possibly the root of the aorta. In the present methodological study, superficial renal cortical blood flow and tissue oxygenation were measured in anaesthetized pigs by laser Doppler flowmetry and by oxygen surface electrode technique. Rapid and profound transient decreases in superficial renal cortical blood flow and tissue oxygenation were found after injection of small volumes of plasma and saline into the left atrium. This response was present also when solutions without microspheres were injected. The reaction was not abolished by careful adjustment of the injectate temperature. When the rapid onset of flow reduction is related to the estimated time of delivery of the bolus with microspheres, the validity of regional blood flow measurements using the microsphere technique within the superficial renal cortex must be seriously questioned.