The polymerization of 1-hexene under high pressures (100-750 MPa) was investigated with nickel-alpha-diimine complex/methylaluminoxane and palladium-alpha-diimine complex/methylaluminoxane as catalyst systems. The catalytic activity of both the nickel and palladium complexes monotonously increased as pressure rose and became two to four times higher than that observed at atmospheric pressure. Palladium catalysts gave poly(I-hexene)s with higher molecular weights under high pressure, whereas nickel-catalyzed high-pressure polymerizations gave polymers with higher molecular weights only at rather low monomer concentrations. The living-like character in the palladium-catalyzed polymerizations was somewhat enhanced under higher pressures, whereas the nickel-catalyzed polymerizations under high pressures were not living. More branches were found in the polymers produced by nickel catalysts at higher pressures. The chain-transfer reaction seemed to be accelerated by the high pressure in the nickel-catalyzed reactions, although this was not apparent in the palladium-catalyzed reactions. Dimers formed and were accompanied by high molecular weight polymers when nickel catalysts were used under high pressures and at high monomer concentrations. The possibility that very congested five-coordinated species act as key intermediates for the dimerization is discussed.