The evolution of pulses in nonlinear single-mode fibres is investigated by means of direct numerical integration of the nonlinear Schrödinger equation and by numerical solution of the associated Zakharov-Shabat scattering equations. For pulses having an initial chirp, we identify and calculate critical values of the chirp. Loss in the fibre is shown to lead to extinction of the soliton character after a finite propagation distance. Phase noise in the initial pulse leads to a jitter in the pulse propagation time.