Neonatal capsaicin treatments (25 or 50 mg/kg, 12, 24, or 48 hr after birth given subcutaneously) were applied in order to follow by immunocytochemical techniques the postnatal development and plasticity of the serotonergic system in the dorsal horn of the rat spinal cord. Two markers of the lesions of C primary afferents induced by capsaicin were tested by immunocytochemical detection: substance P and calcitonin gene-related peptide (CGRP). We show that the internal part of substantia gelatinosa (lamina Ili) which does not contain serotonergic fibers in intact or vehicle-treated rats is invaded within a few days after capsaicin treatment by serotonergic fibers apparently sprouting from the deepest laminae. Moreover, these fibers often establish axodendritic synapses while synapses are rare in intact animals in the whole dorsal horn. This reorganization is stable whatever the dose of capsaicin used or the moment chosen for its injection. On the other hand, while lesions of substance P-ergic fibers appeared quite stable, partial recovery of CGRP innervation was found after 3 to 6 months, especially with the low dose of capsaicin. We discuss the ability of the serotonergic system innervating the dorsal cord either to find new targets or to fill vacated sites when one of its putative targets is removed.