In order to evaluate the effects of fusimotor elimination on the expression of myosin heavy chain (MHC) proteins in intrafusal fibres, we compared the muscle spindles in hind limb muscles of 3- to 6-week-old rats de-efferented at birth with those of their litter-mate controls. Serial sections were labelled with antibodies against slow tonic, slow twitch, fast twitch and neonatal MHC isoforms, against synaptophysin, the neurofilament 68 kD subunit and laminin. We found that de-efferented intrafusal fibres differentiated, as in normal spindles, into nuclear bag and bag fibres both containing predominantly slow MHC, and nuclear chain fibres that contained fast and neonatal MHC. In both de-efferented and control intrafusal fibres the same MHCs were stained; the degree and extent of staining, however, varied. Both types of de-efferented bag fibres displayed a high content of slow tonic and slow twitch MHC along most of the fibre length, in contrast to the prominent regional variation in control bag fibres. In their encapsulated regions, the de-efferented bag fibres were more similar to each other in their reactivity to anti-fast twitch and anti-neonatal MHC antibodies than the control bag fibres. In these aspects they resembled more closely the bag fibres of newborn rats. The differences might be due to an arrest of "specialization" in the regional expression of the different MHC isoforms. Chain fibres developed MHC patterns identical to those of control spindles with all the antibodies used, even though they differentiated from the beginning in the absence of motor innervation. The structural differentiation of the capsule and sensory innervation in de-efferented muscle spindles, as shown by anti-laminin, anti-synaptophysin and anti-neurofilament staining, did not differ from the controls. We conclude, in agreement with previous studies, that the sensory innervation plays a key role in inducing and supporting the differentiation of intrafusal fibres and the specific expression of their MHC. However, we also show that motor innervation and/or muscle function seem to be necessary for the diversity in the expression and distribution of different slow and fast MHC isoforms in the bag and bag fibres.