To study the phenotypic specificity of S-loop and insulin-like growth factor I1 (IGF-11) for developing monoamine neurons, serotonin (5-HT) neurons from the embryonic day 14 (E14) rostra1 raphe or dopamine (TH) neurons from the substantia nigralventral tegmental area were cultured for 3 days in vitro (3 DIV) in the presence of these factors. Neuronotrophic effects were analyzed by computer-assisted morphometry of 5-HT and TH-immunoreactive neurons. S-loop and IGF-I1 differentially regulated the growth of 5-HT and TH neurons but did not affect their survival. S-loop significantly increased several parameters of neurite outgrowth by 5-HT neurons but inhibited the spatial extent (field area) of TH neurites. IGF-I1 promoted growth of cell bodies of both phenotypes, but only stimulated neurite outgrowth by TH neurons. S-1OOp and IGF-I1 differentially affected the number of GFAP immunoreactive cells from raphe and substantia nigra, but these effects did not correlate with the specificity of neuronotrophic effects. S-loop and IGF-I1 immunoreactivities were expressed in glial cultures derived from the same brain regions, raising the possibility that these factors have autocrine effects on glia as well as paracrine actions on neurons. The results of this study suggest that specificity of neuronotrophic factors for particular embryonic neurons may be correlated with their neurotransmitter phenotype.