Patterns of activation (A) and perhaps repolarization (R) depend on myocardial fiber structure and intercellular resistance, parallel and perpendicular to fiber orientation. Gray scale maps of A and R were measured from Langendorff preparations of left guinea pig ventricles stained with a voltage-sensitive dye (di-4-ANEPPS). Action potentials (124) were recorded from syncytia (6x6 and 12x12 mm) under SA node control, or stimulated at the edges (4) of a patch viewed with a photodiode array. At the end of the runs, muscles were marked with ink, fixed, sectioned every 5 pm as a function of depth, up to 1 mm. Fiber axis rotated less than 15 degrees in depth for the first 0.5 mm of epicardium and was aligned with respect to optical maps of A and R. Fast and slow A pathways matched respectively the parallel and perpendicular orientations of the fiber axis. R did not follow the fast axis of fiber orientation, but appears to travel either transverse or 45 degrees to it. R typically occurred at the apex of the ventricle suggesting that these cells have intrinsically shorter action potential durations (APD's). Under SA node control, Purkinje fibers accelerated the conduction velocity of the A process 4 fold over electrical pacing. The average velocity of R, however, remained the same whether SA node or electrically paced, demonstrating that Purkinje fibers do not drive the R process. During hypoxia, A patterns and conduction velocity remained stable, but APD's decreased dramatically within 10 minutes and the pattern of R become random and its velocity decreased. Thus R is also an active process, dependent on cell-to-cell coupling and highly susceptible to hypoxia.