The dynamical evolution of open star clusters has been followed by numerical N-body simulations. These computations have been done considering both the interactions between stars in the cluster, and the effect of a linearized galactic tidal field, assuming circular orbits in the solar neighbourhood. Models have been studied for 250, 500 and 1000 stars, and two initial mass spectra (α=2.35 and α=2.75). We adopt a mean mass and a mass ratio of 50:1 between the heaviest and the lightest star. A simple scheme for instantaneous mass-loss from stellar evolution (neutron stars and white dwarfs) is incorporated into the models using published parameters. The models with 250 stars are found to be relatively insensitive to the adopted value of (the virial theorem scale factor) in the range of pc. The disruption half-lifes (T1/2) are about 1.7 × 108y for models with 250 stars, and 4.6 × 108 y for 1000 stars and α=2.75. Preliminary work including successive encounters of clusters with interstellar clouds appears to give shorter disruption times. Two effects play an important role in the dynamical evolution of the cluster: the formation of one or two massive and energetic binaries (≃50 per cent of the total energy) during the first few crossing times, and their disruption by mass-loss.