A method was developed to estimate three-dimensional (3-D) conduction velocity vector fields in cardiac tissue. The technique was applied to sinus rhythm and pacing mapped with plunge needles at 396-466 sites in the canine myocardium. The speed and direction of propagation was found from polynomial "surfaces" fitted to space-time (x,y,z,t) coordinates of cardiac activity (a generalization of a prior 2-D method). Conduction velocity under ischemic conditions was examined after occlusion of the circumflex (CFX). Estimates of conduction speeds during pacing and ischemia were slower than those found during normal sinus rhythm. Vector field directions were also found to differ between different types of beats. The method was validated on simulated 3-D plane and spherical waves. The proposed approach to computing velocity vector fields provides automated qualitative and quantitative description of electrical activity in 3-D tissue.