When air containing particles is sucked into a bulky sampling head, idealized in two dimensions by a horizontal cylinder with an orifice in the under side, a critical rate of suction exists before particles are sampled. The actual trajectories of particles influenced by gravity and the flow into the orifice, but neglecting inertia effects, are calculated for different rates of suction.The efficiency of sampling rises from zero to unity as the flow increases from the critical value to infinity. In the case of a head and orifice with symmetry about a vertical axis, it is shown that the efficiencies of sampling particles of different sizes are independent of the size of the orifice and the thickness of the walls around it but can be calculated in terms of the rate of suction and the area of the horizontal projection of the head.