Insulin-like growth factors (IGF) I and II are polypeptides with both growth-promoting and insulin-like metabolic effects. The developmentally specific expression of IGF I and II in the nervous system implies a role for these growth factors in neuronal growth and differentiation. In the present study, we analyzed IGF and IGF receptor mRNA transcripts from two related human neuroblastoma cell lines, SH-SY5Y and SK-N-SH. These cell lines provide a good in vitro model of neuronal development. Northern analysis of total RNA from each cell line revealed three IGF II mRNA transcripts (6.0, 4.8, and 1.8 kb), and one mRNA transcript each for the type I (11.0 kb) and type II (9.4 kb) IGF receptors. The size distribution of these multiple transcripts is similar to that found during normal human fetal development. These results establish both cell lines as good in vitro models for investigating the mechanisms which underly IGF gene expression during nervous system development.