Methods from nonlinear dynamics were applied to test the hypothesis that the dynamics of sustained atrial fibrillation (AF) is modified by the class Ic drug cibenzoline during pharmacological conversion. The experiments were performed in conscious goats in which sustained AF was induced by continuous maintenance of AF via programmed electrical stimulation. Data were collected from electrophysiological experiments in five goats to terminate sustained AF by continuous infusion of cibenzoline. Sets of five unipolar epicardial electrograms of one minute duration were recorded from the left and right atrial free wall during sustained AF (control), and at three episodes during infusion of cibenzoline, when the mean AF interval had been prolonged to 25%, 50% and 85% with respect to control. Ventricular far-field potentials were removed from atrial electrograms by a coherent averaging procedure. Using the Grassberger-Procaccia method, the dynamics of the local atrial electrograms was investigated by estimating the (coarse-grained) correlation dimension and correlation entropy from the correlation integral. The results were related to a recently proposed classification (types I-III) of AF based on the degree of complexity of atrial activation patterns. The coarse-grained correlation dimension D(cg) and entropy K(cg) indicated that sustained AF corresponded to type II. During drug administration the coarse-grained parameters were not significantly different from control. Scaling regions in the correlation integral were observed after infusion of cibenzoline (3 out of 5 goats) suggesting that the drug introduced low-dimensional features (type I) in the dynamics of AF (correlation dimension D ranging from 2.8 to 4.4 and correlation entropy K from 1.6 to 6.2 nats/s). Sinus rhythm recorded shortly after cardioversion was very regular (D<2 and K<3 nats/s). The hypothesis that the electrograms during AF and sinus rhythm were generated by a static transformation of a linear Gaussian random process was rejected using a test for time reversibility. The nonlinear analysis revealed that cibenzoline does not significantly alter the dynamics of sustained AF during pharmacological conversion other than a slowing down of the atrial activation and a somewhat increasing global organization of the atrial activation pattern. The sudden change in the dynamical behavior at cardioversion suggests a mechanism that is reminiscent of a bifurcation. (c) 1997 American Institute of Physics.