We have investigated the dynamic relationship between fiber structure and deformation of the epicardium in the canine right ventricle. We have reconstructed this deformation as the motion undergone by groups of three radiopaque markers implanted close together in small epicardial regions. As reported earlier, this motion has a stretch and a rotation component. The stretch relates to shortening in the epicardial segment, whereas the rotation describes how this segment changes orientation during the heart cycle. If one postulates this segment as attached to a myocardial spiral, then one can interpret the systolic shortening of the ventricular axis as a compression of the spiral with decrease in pitch and the diastolic lengthening as an extension with increase in pitch. If one further postulates that the epicardial segment is attached to a right-hand spiral, then decreases in pitch imply local clockwise segment rotations, and increases, local counterclockwise rotations. From the motion of the implanted markers, we have found that a small segment ion the midfree wall of the right ventricle twists first locally clockwise as the ventricular axis shortens and later counterclockwise as the axis lengthens. We conclude that deformation of the epicardium is the result of a helical fiber arrangement that is characteristic of the endocardial half of the wall. Near the apex we observed the segment rotation to be reversed: counterclockwise during systole and clockwise during diastole. This observation implies a left-hand helix, which is the path of the apical epicardial fibers, suggesting that the endocardial half of the wall is no longer predominant here.