Presented here is a description of an ultrahigh-energy hydrogen thyratron/SCR bidirectional waveform research defibrillator having a sinusoidal voltage source for inducing fibrillation and three pulse generators for generating defibrillatory waveforms. The first pulse generator uses an 18 kJ capacitor bank at 0. 8. , 1. 6 or 2. 4 kV discharged through the chest by two series-connected silicon-controlled rectifiers (SCRs). Two other series-connected SCRs in parallel with the bank terminate the discharge. The second pulse generator uses an 18 kJ bank at 5, 10 or 15kV. Ceramic-enveloped hydrogen thyratrons in series with the chest initiate the discharge, and in parallel with the bank terminate the discharge. The third pulse generator supplies a reverse-current pulse when used with either the first or second pulse generators to produce bidirectional waveforms. An 18 kJ bank at 2. 5, 5 or 10 kV is discharged by an SCR chain in series with the chest. The discharge is terminated by an SCR chain in parallel with the bank.