Is fibrillation chaos? Ventricular fibrillation is examined to determine whether it is an instance of deterministic chaos. Surface ECGs from dogs in fibrillation were used to generate a state space representation of fibrillation. Our analysis failed to identify a low-dimensional attractor that could be associated with fibrillation. The results suggest that fibrillation is similar to a nonchaotic random signal. We note, however, that such random-looking but nonchaotic behavior can also be generated by a nonlinear deterministic system. 