A reflectance spectrophotometric measurement was developed to analyze the microcirculation of the skin in real time. The relative absorption (RA) spectra were obtained from skin tissue 10 times per second using a spectro-multi-channel-photodetector system. In this system, white light was projected onto the skin of the back of the anesthetized rat and the spectrum of the reflected light between 450 and 643 nm was analyzed. Two absorption peaks at wavelengths of about 540 and 577 nm were observed, corresponding to the absorption peaks in the oxyhemoglobin (HbO2) spectrum. Either phlebotomy or the i.v. injection of norepinephrine (NE) reduced the RA spectrum. Peaks of the reduction in spectrum [(RA spectrum before the treatment) - (RA spectrum after the treatment)] were observed at about 540 and 577 nm. Injection of NE (0.75-48 ng/100 g B.W., i.v.) reduced the RA value at 577 nm dose-dependently, suggesting a decrease in the skin blood flow due to vasoconstriction. In addition, the content of HbO2 and capillary permeation were measured at the same time after the i.v. injection of Evans blue (EB) dye. As indices of HbO2 content and capillary permeation, RA changes at 540 and 610 nm (the latter is an absorption peak in the EB spectrum) were measured in real time, respectively. Both RA values increased dose-dependently after the intradermal injection of histamine (0.3-100 micrograms/site), suggesting the presence of vasodilation and an increase in permeability. The time at which the RA value for HbO2 reached a maximum was shorter than that for EB. These observations suggest that the method described here can detect changes in the HbO2 content and permeation of skin microcirculation at the same time, and in real time.