An experimental investigation of the theological response of magnetorheological suspensions subjected to step changes in applied magnetic field strength at fixed shear rate is reported. For small applied field strengths, the shear stress increases rapidly to a steady value. Above a critical field strength, the rapid initial increase in shear stress is followed by a slow, transient increase in stress. The critical Mason number corresponding to the critical magnetic field strength at the onset of this transient depends on the particle volume fraction Lis well as the shear rate. This is in contrast to a previous analysis where the critical Mason number was predicted to depend on only the particle volume fraction. The discrepancy is attributed to colloidal forces that are significant in our experimental system, but were not included in the analysis. Further comparison with the previous analysis requires either including the effects of colloidal forces, or performing experiments with systems in which colloidal forces are not important. (C) 2005 The Society of Rheology.