We have previously described insulin to be synthesized "de novo" within the fetal rat brain and that brain endogenous insulin [I(n)] promoted neurofilament distribution within fetal neurons. In this study, we investigated the role of I(n) in neuron axonal growth. Rat fetal brain stem cells from 16-day gestational age were cultured in an IFDM and treated with an insulin antibody. In addition, the cell cultures were also treated in defined medium with the addition of: 5 ng, 20 ng or 100 ng/ml of insulin or 100 ng/ml insulin-like growth factor 1 (IGF-1). The neuron cell cultures were studied at 1 and 3 days of incubation. The presence of preproinsulin mRNA and insulin immunoreaction confirmed the "de novo" synthesis of insulin by the fetal neuron cell cultures. Axonal growth was similar by day 1 of the study in all the media, but in insulin medium containing 100 ng/ml of insulin the axonal length was significantly longer. By day 3 of incubation I(n) promoted axonal growth. Treating the neurons with an insulin antibody confirmed these findings, with a significant decrease in axonal length (p<0.05). The treatment with different concentrations of exogenous insulin did not promote axonal growth beyond I(n) by day 3 of incubation. IGF-1 did not promote axonal growth by day 3 of incubation. In summary, I(n) may promote axonal growth during brain development.