Air embolism of the brain in rabbits pretreated with mechlorethamine. Infusion of 400 microliters air into the left internal carotid artery of five anesthetized rabbits caused transient pial arteriole air embolism, an immediate 41.9 +/- 0.8% dilatation of the embolized vessels, suppression of the cortical somatosensory evoked response to 29.4 +/- 2.7% of baseline, and a progressive decline in ipsilateral cerebral blood flow (measured by hydrogen clearance) to 46 +/- 4.1% of baseline after 2 hours. These values were significantly different from those at baseline and from the responses of 10 control rabbits given equivalent intracarotid saline infusions. Twelve other rabbits were made leukopenic by treatment with 1.5 mg/kg i.v. mechlorethamine 72 hours prior to study. Mean +/- SEM leukocyte count decreased from 6,320 +/- 73/mm3 to 1,890 +/- 66/mm3 without any change in the leukocyte differential or erythrocyte and platelet counts. Intracarotid infusion of saline into seven of the leukopenic rabbits caused no changes. In the other five leukopenic rabbits, infusion of 400 microliters air caused air embolism but did not produce the anticipated declines in cerebral blood flow or the cortical somatosensory evoked response, both of which remained indistinguishable from baseline values and responses in the seven saline-treated leukopenic controls. Similarly, air-embolized arterioles showed nonsignificant dilatation in leukopenic rabbits. Our data suggest that the decreases in both cerebral blood flow and brain function seen after air embolism require the presence of leukocytes. 