The results of a research project aiming to visualize cardiac anatomy in a 3-dimensional form for surgical planning are presented. Based on electronic data processing within a local area network environment, serial slices of CT- and MRI-machines were used to construct a 3-dimensional data cube that was illuminated by calculated sources of light. Light rays were traced through the entire "data volume". Mathematically following each pathway of light through the space, the intensity of changes along this path were calculated. The results of applying this "Heidelberg Ray-tracing Technique" to cardiac anatomy are 3-dimensional "computer movies" that appear on any workstation within a computer network. Using special software, the surgeon can "walk" in any direction through the heart or he can break it into two or more parts in order to analyze regions of interest in detail. Even small structures such as papillary muscles, bifurcations, coronary arteries and cusps of cardiac valves become visible. This new technique may enable the surgeon to open the heart prior to surgery on the computer monitor working with a visualized model that corresponds to the visual experience of his daily intraoperative practice.