The effect of viscosity of barium sulphate suspensions on coating was assessed by glass slides as in vitro, and the rat as in vivo models. It was found that below a certain viscosity (as measured by a cup viscometer) coating was impaired. As increasing viscosity is associated with increasing bubble retention and, eventually, when very high viscosities are reached, poor surface flow of suspension, it is recommended that the viscosity of the medium employed should be kept within a narrow range, approximately equal to 70% w/v Baritop, 80% Micropaque, 115% E--Z-paque, 150% Barosperse, or 200% HD 200.