The ab initio method at the Hartree-Fock level has been used on two zirconocene systems, the unbridged Cp(2)ZrCH(3)(+) and the bridged (Si(CH3)(2))Cp(2)ZrCH(3)(+) compounds. For each complex the insertion of an ethylene molecule has been followed from reactant to product through the corresponding transition states. The calculated activation energies are of the order of magnitude of those obtained experimentally, explaining the higher activity found for the unbridged catalytic complex compared to the bridged. The geometries are in agreement with other studies and comparable with those derived by crystallographic methods. The alpha-agostic interaction observed in the reactants as well as in the transition states and resulting products confirms that the Brookhart-Green mechanism is followed for the polymerization reaction.