The effect of entrance section geometry and superposed sources of disturbance on the minimum Reynolds number marking the breakdown of laminar flow was investigated experimentally. Twelve different flow configurations were employed throughout the course of the study. It was found that the presence of stationary sources of disturbance in the upstream plenum chamber and in the entrance section does not have a significant effect on the breakdown Reynolds number. A pulsating disturbance has a greater influence on the breakdown of the laminar regime, as does asymmetric entrance section geometry. The lowest value of the breakdown Reynolds number encountered in these studies was 2200, corresponding to a disturbance source situated within the channel itself.