The effect of a conditioning lesion on the time-course of axonal sprout formation after a subsequent testing lesion was evaluated in myelinated axons of the rat sciatic nerve. Transmission electron microscopy of longitudinal nerve sections was used to examine nodes of Ranvier located 200-500 micron proximal to the testing lesion. The conditioning lesion was a cut of the tibial nerve at the ankle; the testing lesion, made 2 weeks later, was a crush of the sciatic nerve at the hip. Sprouts were defined as unmyelinated evaginations of the nodal axolemma that (1) had reached the basement membrane of the Schwann cell, and (2) were located between the myelin sheath of the distal paranode and the basement membrane. Photomicrographs of the nodes at 9, 18, and 27 hours after the testing lesion were assigned to one of seven categories: normal, retracted, myelin degeneration, axonal degeneration, type A sprout formation (cytoskeleton absent), type B sprout formation (cytoskeleton present), and type B sprout degeneration. By 9 hours after the testing lesion, type B sprout formation was found in 9% of the nodes in control nerves (testing lesion alone) and 33% of those in conditioned nerves (P less than .01). A 33% incidence of type B sprout formation was not reached in control nerves until 27 hours after the testing lesion. Since the conditioning lesion was located 50 mm distal to the testing lesion and did not induce neuronal death, earlier sprout formation can be attributed to a neuronal response to the conditioning lesion rather than to a putative factor that arises from pre-degenerated fibers and acts on newly formed sprouts.