Thirteen male Sprague-Dawley rats were anaesthetized with halothane and catheters were placed in the femoral artery and left ventricle. The animals breathed spontaneously through a tracheostomy throughout the investigation. After the surgical preparation, the inspired halothane concentration was maintained at 1.5 vol% (F/O2 = 0.3). After a 30-min stabilization period, strontium-85 radiolabelled microspheres (15 +/- 1 micron) were injected to the left ventricle for determination of cardiac output and regional blood flows. In six rats, nitrous oxide was then substituted for nitrogen in the inspired gas mixture. Ten min thereafter, 141Ce-labelled microspheres were injected and the haemodynamic measurements were repeated in all animals. The administration of the nitrous oxide to halothane-anaesthetized rats resulted in a decrease in cardiac output and decreases in blood flow to kidneys, liver, small bowel and spleen. Cerebral perfusion was increased.