Erythrocyte filterability and several hematological parameters were determined in the blood samples obtained from the carotid artery and internal jugular vein of guinea pigs during neurosurgical laser application. Erythrocyte filterability in the blood obtained from the internal jugular vein during laser irradiation was significantly impaired. Intravascular hemolysis was also evident as hematocrit value and red cell counts were decreased, while hemoglobin value remained unchanged. Studies in the control group confirmed that the surgical procedure without laser application had no similar effects. It is proposed that the hemorheological alterations under the influence of surgical laser application may play role in certain complications of laser surgery.