This work presents a technique and develops an apparatus that allows the application of homogeneous external magnetic fields (parallel or perpendicular to the deformation axis) to a fluid sample undergoing extensional flow kinematics while measuring the filament thinning using the commercial version of the capillary breakup extensional rheometer (Haake (TM) CaBER (TM) 1, Thermo Scientific). We also present innovative rheological measurements of several commercial ferrofluids (FFs) and one magnetorheological fluid (MRF) under uniaxial extensional flow. The experimental results demonstrate that FFs exhibit a Newtonian-like behavior in the absence of magnetic fields. When a magnetic field is applied perpendicular to the extensional flow, no significant effects are observed similar to shear experiments. However, when the external magnetic field is aligned with the extensional flow, the filament takes longer to break up but otherwise behaves as a Newtonian fluid. In the case of the MRF, due to the higher concentration of particles and larger particle size, the differences in the extensional behaviors are much more dramatic regardless of the orientation of the magnetic field compared to the case when no magnetic field is applied. (C) 2015 The Society of Rheology.