Trophic effect of iron-bound transferrin (FeTf) on the total content of acetylcholine receptors (AChRs) and the specific activity of AChRs in innervated and denervated skeletal muscle was investigated in vivo. The right ischiadic nerves of 15 rats weighing 160 g were transected. FeTf (1.2 mg/ml) was injected daily into bilateral crural muscles of rats for the following 11 days. Control groups received injections of saline or no treatment. FeTf significantly increased the total content of AChRs and the specific activity of AChRs in innervated and denervated muscle compared with control groups (P less than 0.001). This result shows that intramuscular injections of FeTf may be useful for the treatment of disorders of neuromuscular transmission.