The aging of spinal cord alpha motoneurons was explored in old cats with intracellular recording techniques to determine the basic membrane properties of these neurons and their monosynaptic response following activation of group Ia afferent fibers. The conduction velocity of the motoneurons' axons decreased in old animals (14 to 15 years of age) compared with adult controls (1 to 3 years of age). The input resistance of the motoneurons increased in the old cats; no change occurred in the resting membrane potential or spike amplitude. There was a reduction in the delay between the initial segment and the somadendritic components of the antidromic spike. The half-width duration of the monosynaptic EPSP in the old cats increased, but its amplitude did not change. These data indicate that a host of different membrane properties of spinal cord motoneurons and their Ia-monosynaptic input are affected by the aging process. Analysis of the results suggests that the degradation of neuronal processes occurs in all motoneurons rather than preferentially affecting a specific population of motoneurons.