The recent discovery1 of rapidly pulsed radio sources and the further detailed observations2 of the source CP 1919 + 22 raise remarkable problems. Undoubtedly the theory which requires the least change in our concepts is that of a pulsating white dwarf. As long ago as 1952, Mestel3 pointed out that nuclear fuel in the outer regions of a white dwarf can cause an explosive instability. There is therefore no difficulty in understanding how an oscillatory state might be set up. It is also possible that oscillations could be self-sustaining.