The response of developing muscle spindles to denervation was studied by sectioning the nerve to the medial gastrocnemius muscle of rats at birth. The denervated spindles were examined daily throughout the first postnatal week for changes in ultrastructure and expression of several isoforms of myosin heavy chain (MHC). Each of the three different types of intrafusal muscle fiber exhibited a different response to denervation. Within 5 days after the nerve section nuclear bag2 fibers degenerated completely; nuclear bag1 fibers persisted, but ceased to express the 'spindle-specific' slow-tonic MHC isoform and thereby could not be differentiated from extrafusal fibers; nuclear chain fibers did not form. The capsules of spindles disassembled, hence spindles or their remnants could no longer be identified 1 week after denervation. Neonatal deefferentation has little effect on these features of developing spindles, so removal of afferent innervation is presumably the factor that induces the loss of spindles in denervated muscles. Degeneration of the bag2 fiber, but not bag1 or extrafusal fibers, reflects a greater dependence of the bag2 fiber than the bag1 fiber on afferent innervation for maintenance of its structural integrity. This difference in response of the two types of immature bag fiber to denervation might reflect an origin of the bag2 fibers from a lineage of myogenic cells distinct from that giving rise to bag1 or extrafusal fibers, or a difference in the length of contact with afferents between the two types of bag fiber prior to nerve section.