Gravitational convection in a horizontal layer of magnetic colloid heated from below has been studied experimentally. Two series of experiments, differing from each other in the initial state of the colloid, have been conducted. In the first series of experiments the colloid was initially in the equilibrium state, in the second one the fluid was premixed by convection. The oscillatory regime of convection and the regime of large-scale eddy motion were found near the threshold of equilibrium stability. The occurrence of these regimes of convection is explained by the influence of two competing effects: thermal diffusion and barometric separation of magnetic particles. (C) 2015 Elsevier Ltd. All rights reserved.