We report a method to measure the viscosity of microL volumes of biofluid obtained from threadlike structures (NTSs) on the surfaces of mammalian (rabbit) internal organs. The fluid was mechanically microextracted in situ from NTSs on the organ surfaces by a glass capillary connected to an extractor. From the Brownian motion of the 0.8+/-0.1microm diameter granules in the extracted fluid, the fluid viscosity was determined to be 1.4+/-0.1mPa s at room temperature. This viscosity is comparable to the viscosity of rabbit blood plasma.