The variation of the magnetic field at the centre of a sunspot is studied for spots of lifetime 30 and 55 days. The study shows that, if the magnetic field is produced by the darkening of the spot, a time at most of order one day is taken to establish the field. Theory indicates that 300 years is required for the growth or decay of the field. It is concluded that the sunspot field must exist both before and after the visible lifetime of the spot.