Abstract: A high-temperature, high-pressure process for polymerizing ethylene in a reactor system having at least two reaction zones with a catalytic system including an activator (a), a compound (b), and a compound (c). Compounds (b) and (c) are injected into different reaction zones. Activator (a) is selected from hydrides and organometallic compounds of a metal of groups I to III of the Periodic Table. Compound (b) is represented by the formula(TiCl.sub.a) (MgCl.sub.2).sub.y (AlCl.sub.3).sub.z (RMgCl).sub.bin which2.ltoreq.a.ltoreq.3, y.gtoreq.2, O.ltoreq.z .ltoreq.1/3, O.ltoreq.b.ltoreq.1and R is an aliphatic or aromatic hydrocarbon radical.Compound (c) is represented by the formulaTiCl.sub.3 (AlCl.sub.3).sub.w (E, TiCl.sub.4).sub.xin which O.ltoreq.w.ltoreq.1/3, O.ltoreq.x.ltoreq.0, 03 and E is a diisoamyl- or di-n-butylether.The amounts of (a), (b), and (c) are such that the ratio Al/Ti is between 1 and 10 in each reaction zone.