Buoyancy-driven motion and breakup of viscous drops in sinusoidally constricted channels are studied computationally using a finite-volume/front-tracking (FV/FT) method. Computational results are first compared with the available experimental data and then the conditions for a drop breakup are examined. The effects of the channel geometry, drop size and Bond number on drop breakup are investigated. It is found that the present computational results are in a good agreement with the available experimental data and drop breakup occurs in various modes depending on the channel geometry, the relative drop size and Bond number. It is observed that a drop breaks up into two or more drops when its size is larger than a critical value. It is also found that large drops undergo successive breakups to produce monodispersed small droplets. Critical non-dimensional drop size and critical capillary number for the onset of drop breakup are reported over a wide range of geometrical parameters and Bond numbers. (c) 2006 Elsevier Ltd. All rights reserved.