Treatment with buprenorphine represents a promising alternative for pregnant opioid addicts but there is a need to understand potential effects on nervous system development. We previously showed effects of perinatal exposure to buprenorphine on axonal caliber and myelination in 26-day-old rat corpus callosum. These changes, detected at the end of rapid brain myelination and accompanied by earlier oligodendrocyte maturation, suggested interference with mechanisms coordinating axonal growth and myelination. To better understand buprenorphine actions and to establish whether these effects extend to the spinal cord, we analyzed the corpus callosum and corticospinal tract at 16 days of age, just before the peak of myelination. Our results point to an important role of the opioid system in regulating early axo-glial interactions coordinating axonal growth and myelination. Moreover, in addition to reinforcing previous findings in the brain, we showed for the first time that these effects are also exerted in the spinal cord.