In the neo-classical theory of transport process in a toroidal plasma with long mean free path, the value of the poloidal rotation is determined by ambipolarity. This paper investigates the time-dependent phase during which this rotation develops. Diffusion arises from resonant particles, for which v∥ ≃ Er /Bφ. The time a particle spends in resonance, and hence its radial displacement, is now determined by the rate of change of Er, rather than collisional scattering. The time for Er to approach its quasi-stationary value is comparable to the mean time for an ion to travel the connexion length. This is short compared with typical experimental durations. The mass flow also builds up parallel to the magnetic field, but its contribution to the poloidal flow is small compared with the Er /B electric drift.