Vasoactive drugs are normally utilized to elevate mean artery pressure and maintain adequate organ perfusion in clinical treatment. During the injection, morphological changes and the subsequent oxygen supply alteration in the brain, e.g., possible hypoxia, are prone to introduce serious damage and even dysfunction to the brain. Therefore, multiparameter monitoring of cerebral microvasculature is necessary during drug injection. An optical-resolution photoacoustic microscopy was used to assess the effects of norepinephrine on microvasculature in the brain cortex of mice. In our experiments, the diameter, total hemoglobin (HbT) and oxygen saturation (SO 2 ) of single cerebral microvessels during tail vein injection of norepinephrine were analyzed. Following the injection, vasoconstriction was observed, and HbT and SO 2 were decreased in turn. The vessel diameter and HbT recovered back to the base value without further injection, while the SO 2 remained low throughout the observation period. Arterioles showed more acute constriction but a smaller decline in HbT during the injection compared with venules, while SO 2 in arterioles increased slightly without further drug injection but not in venules. Our results suggested that photoacoustic microscopy may become a new method for early and comprehensive evaluation of the effect of drugs on microvasculature in brain.