Microtubule associated proteins play a central role in the control of axon growth. We have used immunohistochemical techniques to establish which microtubule-associated proteins are present in the rat hindlimb spinal cord, dorsal root ganglia and peripheral nerves during axonal growth during embryogenesis, in adulthood, and during regeneration of crushed sciatic nerves. During embryogenesis microtubule-associated protein-1b and tau are present in all neurons and axons, microtubule-associated protein-2 is present in neurons but not in axons, and there is no microtubule-associated protein-1a. In adults, microtubule-associated protein-1a and microtubule-associated protein-1b are present in all sciatic nerve axons and in motor and dorsal root ganglion neurons. Tau, in its adult form, is present in many fine probably sensory axons, but not in most larger axons, and in motor and sensory neurons. Microtubule-associated protein-2 is present only in neurons. During regeneration the pattern of microtubule-associated protein expression retains the adult pattern. All regenerating axons contain microtubule-associated protein-1a and microtubule-associated protein-1b, none contain microtubule-associated protein-2, and a subset of fine axons contain tau. There is no detectable change in microtubule-associated protein expression by motoneurons. While axons are clearly able to regenerate without either microtubule-associated protein-2 or tau, tau containing axons appear to regenerate faster than those which lack it. It is possible that the failure of neurons to recapitulate the embryonic pattern of microtubule-associated protein expression during regeneration could be a reason why regenerative axon growth is slower and less vigorous than axon growth in embryos.