The ultrastructure of regenerating intrafusal and extrafusal fibers was studied 18 h to 30 days after heterochronous isotransplantation, in which bupivacaine-treated extensor digitorum longus (EDL) or soleus muscles from early postnatal rats were intramuscularly grafted into EDL muscles of adult inbred recipients. As in other models of mammalian muscle regeneration, surviving satellite cells gave rise to presumptive myoblasts, multiplying within the preserved basal lamina tubes at day 4 after grafting. Myoblasts fused to form myotubes with central myonuclei by day 6 after grafting. Extrafusal myotubes differentiated into thin muscle fibers by day 8, which progressively increased in diameter and their nuclei became localized subsarcolemmally from day 13 onwards. The basal laminae of some intrafusal fibers already contained one or more nascent myotubes by day 4 after grafting. Regenerated intrafusal fibers lacked the typical nuclear accumulations and their number varied from one to eight fibers per spindle; additional fibers formed in the periaxial space or between layers of the capsule. Regenerated muscle spindles usually had a thinner outer capsule and a reduced inner capsule and periaxial space. The present study demonstrates that extrafusal and intrafusal muscle fibers degenerate and regenerate after heterochronous isotransplantation in a manner similar to that in standard grafts. However, the time course is slightly different. Degeneration was completed by day 5 after grafting as in free grafts, but the regeneration of extrafusal and intrafusal fibers started 1 or 2 days earlier, apparently because of the rapid and facilitated revascularization from the host muscle compared to that of standard grafts.