A reliable pulse source circuit utilizing two independent but synchronized pulse generators is disclosed. The pulse source circuit maintains output pulses even though one pulse generator fails regardless of the failure mode, i.e., whether the output of the failed generator is stuck high or stuck low. A toggle flip-flop responsive to the presence and absence of pulses from the two generators and its own state outputs controls gating circuitry to generate a sequence of output pulses wherein alternate output pulses occur in response to pulses of one generator and the remaining output pulses occur in response to pulses of the other generator, and if either generator fails, all output pulses occur in response to the pulses of the other generator.