The aim of this study was to determine whether the different morphological classes of pyramidal neurons in layers 2/3 and 5 of rat visual cortex (Larkman and Mason, 1990) have particular electrophysiological properties. Neurons in in vitro slices of rat visual cortex were impaled with glass micropipettes containing horseradish peroxidase (HRP) and studied using current-clamp techniques prior to pressure injection of HRP into the neurons. On morphological grounds, cells stained in layer 2/3 were placed into a single class whereas layer 5 cells were divided into 2 classes. Cells in one of these classes had thick apical dendrites which arborized in layer 1, whereas the apical dendrites of cells in the other class were thinner and did not reach layer 1 (Larkman and Mason, 1990). Despite variation between individual cells of a single class, significant differences were found in the time constants, current/voltage relations, and repetitive firing behaviors of the 3 classes. Burst firing responses to injected current pulses were confined to the layer 5 cells with thick apical dendrites. These results add to those from other areas of the brain demonstrating that the electrophysiological properties of pyramidal neurons are heterogeneous. Furthermore, we have shown that distinctive intrinsic membrane properties of pyramidal neurons in visual cortex are correlated with different morphologies.