The behavior of rolling leukocytes in intact skin microvessels was visualized by selective fluorescent labeling technique. After the proper anesthesia with pentobarbital sodium, leukocyte nuclei of male Lewis rats (n = 16, b. wt. = 100-210 g) were stained in situ with acridine yellow (50 mg/kg-b.wt., i. v. for 15 min). The hindpaw of the animals was gently stretched, and fixed on a plateau to the microscope stage. The hairless part of its nailfold was covered with a thin layer of paraffin oil to reduce the surface reflection, and were observed under an intravital fluorescent video microscope. Final optical magnification obtained at the intensified-CCD camera plane was 125. As the criteria of physiological condition of the animals, mean arterial pressure, heart rate, rectal tempreture, and skin temperature were also monitored during the observation. The injected acridine yellow was taken up selectively by the nuclei of leukocytes, and there was almost no staining of cytoplasms. The fluorescent images of the cells were clear during the period of observation (30-40 min), even after the cessetion of the dye injection. The lobes of the nuclei of the rolling leukocytes were also observable. Heart rate, rectal temperature and skin temperature were within normal range during and after the fluorescent dye injection, while mean arterial pressure decreased after the dye injection. There was no apparent platelet thrombus formation in the microvessels during the observation. In conclusion, it was demonstrated that acridine yellow selectively stains the nuclei of leukocytes which could a promising tool to deferentiatein vivo rolling leukocytes as polymorphonuclear or monomorphonuclear ones.