Serial reconstruction at the EM level of cat retinal ganglion cell dendrites reveals that: (1) the microtubular array is discontinuous, (2) microtubular endings are associated with smooth endoplasmic reticulum (SER), mitochondria, and plasma membrane, (3) individual microtubules always maintain a minimum distance from other microtubules (87 nm), SER (43 nm) and plasma membrane (69 nm), and (5) individual microtubules can 'wander' independent of adjacent microtubules throughout the dendritic volume. These observations, taken with some recent biochemical and immunohistochemical data by other workers, suggest that the microtubules are surrounded by a coat of high molecular weight, microtubular-associated proteins (HMW MAPs), which effectively creates a 90 nm tube around a central microtubular core. Our results suggest that bundles of these 'MAP-tubes' may serve as a major component of the dendritic cytoskeleton in the cat ganglion cells.