Olefins are polymerised in the presence of a solid catalyst component and optionally a cocatalyst. The solid catalyst component is introduced into the polymerisation reactor by suspending the solid component into oil having a viscosity of from 20 to 1500 mPa s. Optionally, the suspension comprises a drag reduction agent dissolved in the oil. The suspension is metered into the polymerisation reactor by using a valveless piston pump.