The pressure drop in a horizontal rotating fluidized bed was measured using slotted and sintered metal cylindrical gas distributors as a function of rotating speed, gas velocity and bed thickness. Experiments were conducted using polydisperse alumina particles and nearly monodisperse glass beads. The pressure drop for the slotted distributor exhibited a much larger pressure overshoot at incipient fluidization than the sintered metal distributor. This behavior was also studied using high-resolution photography. Physically consistent explanations are presented for the observed phenomena. The experimental results are compared to theoretical models available in the literature.