  We present the first 3D hydrodynamical simulations of ram pressure stripping
of a disc galaxy orbiting in a galaxy cluster. Along the orbit, the ram
pressure that this galaxy experiences varies with time. In this paper, we focus
on the evolution of the radius and mass of the remaining gas disc and compare
it with the classical analytical estimate proposed by Gunn & Gott 1972. We find
that this simple estimate works well in predicting the evolution of the radius
of the remaining gas disc. Only if the ram pressure increases faster than the
stripping timescale, the disc radius remains larger than predicted. However,
orbits with such short ram pressure peaks are unlikely to occur in other than
compact clusters. Unlike the radius evolution, the mass loss history for the
galaxy is not accurately described by the analytical estimate. Generally, in
the simulations the galaxy loses its gas more slowly than predicted.
