Trophic factors play important roles for the regulation of several motoneuron characteristics. In this report, we have studied the effects of different trophic factors on the regulation of two substances that have been linked with axon sprouting and regeneration in rat spinal motoneurons, i.e., calcitonin gene-related peptide (CGRP) and growth-associated protein-43 (GAP-43). The expression of both GAP-43 and CGRP is regulated developmentally and after different lesions. In the adult animal a sciatic nerve transection normally leads to an increased expression of both alpha-CGRP and GAP-43. Local administration of insulin-like growth factor-1 (IGF-1), IGF-2, or ciliary neurotrophic factor (CNTF) did not significantly change this response, whereas basic fibroblast growth factor (bFGF) attenuated the lesion-induced upregulation of alpha-CGRP mRNA. None of the studied factors had any influence on the upregulation of GAP-43 after lesion. In vitro, GAP-43 and alpha-CGRP mRNAs could be detected in enriched motoneuron cultures prepared from E14 rat embryos. The GAP-43 expression was upregulated by bFGF, IGF-1, IGF-2, and CNTF. CNTF also upregulated alpha-CGRP mRNA in cultured motoneurons, whereas bFGF had the opposite effect. IGF-1 and -2 did not significantly affect the alpha-CGRP mRNA levels. The decrease in GAP-43-immunopositive neuromuscular junctions (NMJs), seen during normal postnatal development, was attenuated after intramuscular injections of bFGF, CNTF, IGF-2, or CGRP 8-37, a CGRP antagonist. At the same time bFGF decreased and CNTF increased the number of CGRP-immunoreactive NMJs, whereas no difference from control was seen in IGF-2-treated muscles. These results show important regulatory effects of trophic factors on the expression of both GAP-43 and alpha-CGRP in motoneurons. However, it also is shown that the trophic factors used here influence the expression of these two substances in distinctively different patterns. This may have important implications for the understanding of trophic interactions in the spinal motor system and also should be considered in the evaluation of possible effects of CGRP and GAP-43 in motoneurons. The effect of a CGRP antagonist on GAP-43 in muscle indicates a role for CGRP in sprouting-related processes.