A self-powered pacemaker uses the variations of blood pressure inside the heart or a major artery to create a periodic change in the magnetic flux inside a coil. The pressure variations compress a bellows carrying a magnet moving inside a coil. The inside of the bellows is evacuated to a partial or full vacuum, and a spring restores the bellows to the desired equilibrium point, acting against the blood pressure. The current pulses are stored in a capacitor. Eliminating the battery allows dramatic miniaturization of the pacemaker to the point it can be implanted at the point of desired stimulation via a catheter. The invention includes means of compensating for atmospheric pressure changes.