Using RT-PCR, we have investigated expression of isoforms β1 (the axonal isoform) and α2a-2b (the mesenchymal isoform) of neuregulin-1, one of the most important known trophic factors for Schwann cells, in the rat sciatic nerve repaired by muscle-enriched non-nervous conduits (made by a vein filled with fresh skeletal muscle). Repaired nerves were harvested 2, 6 and 13 days post-operatively. Results showed that while muscle–vein combined grafts were enriched in mRNA coding for α2a-2b since the very early regeneration stages, isoform β1 mRNA was not detectable inside the tubes at day 2 and 6 post-operatively while its expression at day 13 was very slight. These results suggest that Schwann cell survival and activity inside a fresh muscle-enriched non-nervous conduit graft (a key factor for successful nerve regeneration along the graft) may be supported by the mesenchymal isoform of neuregulin-1 during very early repair phases, i.e. when axons are still not present along the tube.