A new method that uses the reduced scattering coefficients (micro'(s)) measured in rat's brain tissue in vivo after administration of hyperosmotic solution by bifurcated fiber optic probe is proposed in this paper. 60 SD rats were divided into three groups by randomization method, and then were treated by 0.9% NaCl, 20% Mannitol and 7.5% NaCl through vena caudalis, respectively. The changes of micro'(s) in every rat's local cortex were observed continuously by a bifurcated fiber optic probe in vivo in a mini-invasive way. No changes of micro'(s) were observed in the control group which was given by 0.9% NaCl, while the micro'(s), relative changes of the 20% mannitol group and 7.5% NaCl group increased by 7.3% +/- 1.7% and 12.8% +/- 2.9%, respectively. The results showed that there were significant differences among the three groups (P < 0.05). The micro'(s) of rat's local cortex observed by bifurcated fiber optic probe can be used for shedding light on the anhydration induced by hyperosmotic solution.