It has previously been shown that a rapid, transient and pronounced reduction in superficial renal cortical blood flow can be elicited by injection of 1 ml of plasma into the left atrium of the anaesthetized pig. This renal vascular response might constitute an important error in measurements of renal blood flow by means of the microsphere technique. In the present study we have investigated total renal (ultrasonic Doppler) and superficial renal cortical (laser Doppler flowmetry) blood flow after left atrial injections of saline and autologous blood. The average reductions in total renal blood flow were 4.7 +/- 2.4% (mean +/- s.d.) and 18.4 +/- 12.5% (mean +/- s.d.) for the blood and saline injections, respectively. The flow reduction was more pronounced in the superficial renal cortex with average reductions of 17.4 +/- 26.0% (mean +/- s.d.) and 59.4 +/- 38.5% (mean +/- s.d.) for blood and saline, respectively (laser Doppler flowmetry). Furthermore, two consecutive injections of microspheres were performed. The results from the second injection were compared with those from the first injection. In the superficial renal cortex the second injection indicated a significant reduction in blood flow and an significantly increased heterogeneity in the blood flow in two different tissue volumes. No corresponding differences were found in the juxtamedullary cortex. These results indicate that the renal vascular response to a left atrial injection is detectable as a reduction in total renal blood flow and is most pronounced in the superficial renal cortex. All these results taken together constitute an important error in measurements of superficial renal cortical blood flow by the microsphere technique.