We present a fast, robust and automatic method for computing central paths through tubular structures for application to virtual endoscopy. The key idea is to utilize a medial surface algorithm which exploits properties of the average outward flux of the gradient vector field of a Euclidean distance function the boundary of the structure of interest. The algorithm is modified to yield a collection of 3D curves, each of which is locally centered. The approach requires no user interaction, is virtually parameter free and has low computational complexity. We illustrate the approach on segmented colon and vessel data.