Abstract: An explosive initiator for a parachute rip cord release includes a hammer and firing spring mounted at one end of a cartridge chamber and an aneroid device and trigger mechanism mounted at its other end. A slidable sear extends along the cartridge chamber, connected at one end to the hammer and having a cam connection with the trigger mechanism at its other end. The normally uncocked hammer is manually cocked to slide the sear and thereby move the trigger mechanism into an operative position under control of the aneroid device. As ambient pressure decreases below a predetermined value, the aneroid device releases the trigger mechanism to thereby release its restraint upon motion of the sear and allow the spring to drive the hammer and detonate a cartridge in the chamber. Cartridge detonation drives an actuator piston that is connected to the parachute rip cord release.