Suspensions of small dipolar or magnetic nanoparticles, also referred to as ferrofluids or magnetic fluids, are of significant practical interest as they exhibit unique physical properties. Ferrofluids are “superparamagnetic” and, therefore, their flow and properties can be controlled using only moderate magnetic fields of the order of 10 to 100 mT. However, in the absence of an external magnetic field, ferrofluids have no net magnetization as Brownian motion orients the particles randomly. One important characteristic of ferrofluids is that they flow toward regions of strong magnetic fields and remain fixated there as long as the magnetic field is present. The interaction of ferrofluids with an external magnetic field leads to some interesting rheological properties. Brownian couple both contributes directly and indirectly to the symmetric part of the particle stress. The direct contribution to the particle stress results from the modification of the particle angular velocity by Brownian and magnetic torques.