Abstract Magnetorheological fluids (MRFs), i.e. suspensions of ferri- or ferromagnetic particles in a water or an oil-based carrier fluid, stabilized by a surfactant or polymer, are deemed suitable for fluid flow control in oil and gas recovery operations. This paper reports a laboratory study on the rheology of oil-based MRF and its influence on oil and water flow in cores. Two types of experiments will be discussed: (a) flow of an MRF in a tube and (b) flow of oil or brine in sand-packs previously saturated with a MRF. In both types of experiments, the flow rate and the strength of the magnetic field were varied. Tube flow experiments show that MRFs can be characterized by yield point and, above the yield point, by a shear-thinning behavior. In the presence of a magnetic field the rheological parameters change drastically leading to a gel type of behavior. On one hand, the yield point and the plastic viscosity increase strongly and, on the other hand, the power law exponent decreases and then levels off to a plateau. We propose a simple rheological model conveying these features. On the core side, residual resistance factors (FRR) in the order of a few thousand were obtained under moderate magnetic fields. For a fixed magnetic field, the FRR for oil is lower that that for water by at least an order of magnitude. The data suggest that magnetorheological fluids are efficient flow control systems that could be used to maintain pressure in the well at a certain level (e.g., to prevent a "blowout"or leakoff during drilling), to reduce circulation loss or to reduce water production in hydrocarbon wells.