Most of the microvessels under investigation exhibited obvious vasoconstriction immediately after the injection. Vasoconstriction reached the maximum after 3 min. The microvessesls began to recover after severe vasoconstriction which lasted for 2 to 4 min. Normally, the microvessels recovered back to the resting state 7 min after the injection. However, some microvessels sustained a slight contraction throughout the investigation. Figure 4 illustrates typical changing of microvascular morphology during the observation. Figure 4(a) and 4(c) is the transverse view of the morphological changes of an arteriole and a venule, respectively, during the injection of norepinephrine. The horizontal axis shows the time course, and the vertical axis marks the semidiameter along the center of the microvessels. Figure 4(b) and 4(d) is the plot of the diameter of the arteriole and the venule mentioned above, where the blue line shows the raw data, and the red line shows the tendency of the blue line after the application of a mean filter. The arithmetic mean filter set the center point values to be the average of an 8-pixel neighborhood and signals beyond the end points were set to be 0. Both the microvessels constricted severely immediately after injection, reached a constriction stabilization period after about 2.5 min, and recovered thereafter. There are still some differences between the responses of arteriole and venule. The arteriole constricted from 30 μm to a minimum diameter of 15 μm, whereas the venule constricted from 32 μm to a minimum diameter of 22 μm. The constriction ratio of arteriole and venule, which was defined as 1-minimum diameter/resting state diameter, was 50% and 31.25%, respectively. By the end of the observation, the diameter of the arteriole recovered back to the same as the resting state, while the venule preserved a slightly smaller diameter. Ten arterioles and ten venules were conducted in statistical analysis, respectively. The arterioles constricted to a minimum diameter that is of the resting state, whereas the venules reached only , showing a significant difference (). By the end of the observation, the diameter of arterioles recovered to of the base value, the venules only back to , showing a significant statistical difference (). In the control trials, no changes of diameter were observed throughout the observation, indicating that the observed phenomenon was not the effect of tail vein injection of exogenous reagents. The observations suggested that cerebral microvasculature of healthy mice constricted severely when intravenously injected with norepinephrine, but recovered in a short time without prolonged drug injection thereafter. The arterioles exhibited more intense constriction than venules.