The effect of hydrogen on the polymerization rate of ethylene catalysed with a non-supported, heterogeneous Ziegler-Natta catalyst, is compared to the previously reported rate enhancement effect of hydrogen on the polymerization of propylene with the same catalyst system. It is shown that while hydrogen apparently activates new catalytic site types for the polymerization of propylene, the effect of hydrogen on the polymerization rate of ethylene is practically negligible. Prepolymerizing the catalyst with propylene increases the polymerization rate of ethylene as compared with the rate of ethylene polymerization with non-prepolymerized catalyst, but the effect of hydrogen on the polymerization rate of ethylene with the prepolymerized catalyst remains negligible.