Relationships between axonal diameter, soma size, and axonal conduction velocity were examined in intracellularly recorded pyramidal tract (PT) cells of conscious cats using pressure injection of horseradish peroxidase. Positive linear correlations were found between axonal conduction velocities and axonal diameters as well as between axonal conduction velocities and soma sizes. All PT cells had somata located in layer V. Slow PT cells had high densities of dendritic spines in layer III; however, so did some fast PT cells, making this morphologic feature unacceptable for distinguishing between slow and fast conducting PT neurons.