To investigate the effect of dibutyryl cyclic AMP (dbcAMP) on ventricular fibrillation after coronary reperfusion, the proximal portion of the anterior descending branch of left coronary artery was reperfused 20 min after ligation in 24 cats. McFee X Y Z electrocardiograms were recorded and ventricular fibrillation was analyzed using a fast Fourier transform analysis (FFT). Ventricular fibrillation occurred in 20 of 24 cases. Sixty seconds after the occurrence of ventricular fibrillation, an intracardiac infusion of dbcAMP was administered. Nine of the 20 were defibrillated and converted to sinus rhythm or junctional rhythm after the administration of dbcAMP. The amplitude and frequency of the main power spectrum of the ventricular fibrillation waves were analyzed by FFT before and after the infusion of saline or dbcAMP. In the saline group there was no significant change in FFT. However, in the dbcAMP group, the amplitude increased significantly from 0.036 +/- 0.015 (MV--2) to 0.054 +/- 0.013 (MV--2) (p < 0.01) and the frequency decreased significantly from 4.22 +/- 1.37 (Hz) to 1.33 +/- 0.91 (Hz) (p < 0.01). Those results indicate that dbcAMP increased the amplitude and decreased the frequency of the main power spectrum of ventricular fibrillation analyzed by FFT. These distinctive changes in FFT analysis were associated with defibrillation in 9 of 20 cases.