The origin and propagation sequence of cardiac depolarization in situ requires accurate simultaneous three-dimensional information from multiple sites. Likewise, the mechanism underlying an arrhythmia can often be elucidated by determining the course of impulse propagation through the heart, particularly if information can be obtained from multiple sites simultaneously, allowing analysis of transient or rapidly occurring events. The most significant problem with obtaining such detailed continuous information is the large amount of data storage required as well as the need for rapid analysis. In the present system these problems are overcome by immediate conversion of all electrograms from analog to digital for all subsequent storage and processing. The bipolar electrogram information is acquired from 240 cardiac sites simultaneously at a sampling rate of 2 kHz with continuous and total data storage of up to 60 min. Rapid two-dimensional isochronic maps at multiple depths (effective 3-dimensional information) are presented via computer-generated interactive graphics. System design permits easy expansion to almost 2,000 simultaneous sites. Surgical electrophysiological intraoperative studies in humans are performed at an operating room located 2,000 ft from the computer facility with all communications carried by a fiber-optic link. The system allows both experimental and clinical cardiac mapping from multiple sites from a single cardiac depolarization, minimal redundancy of costly hardware, and direct rapid visualization of all original electrogram data.