Objective  To investigate the long-term morphologic changes of the gastrocnemius muscles of SD rats after botulinum toxin A (BTXA) gel induced paralysis.   Methods  BTXA labeled with

125

I was obtained. The radioactivity measurement was taken to measure the solubility of BTXA in gel and BTXA solution (normal saline as solvent). The right gastrocnemius muscles of 12 SD rats were injected with BTXA gel and the left gastrocnemius muscles injected with normal saline were used as controls. HE staining and 1% gold chloride staining were used to study the

tissue changes 6 months and 12 months after BTXA gel injection and transmission electron microscopy was used to observe the ultrastructural changes.   Results  The radioactivity of

125

I-BTXA gel and

125

I-BTXA solution had no significant differences (P0.05). Histopathologic examination showed no changes in the right gastrocnemius muscles injected with BTXA gel

but ultrastructurally the myopathic changes were clearly visible

like diffuse sarcomere disruption and saroplasmic reticulum expanding. The myofibre degeneration showed no remission 12 months after BTXA gel injection.   Conclusion  BTXA is dissolved in gel evenly. The long-lasting myofibre degeneration in BTXA gel paralyzed muscles may reflect that the paralyzed muscles fail to regain their unique function and recovery of muscle contraction.