Growth of the male external genitalia is primarily regulated by androgens; however, several observations suggest growth hormone (GH) or a GH dependent factor, such as insulin-like growth factor-1 (IGF-1), might also be involved. It is hypothesized that testosterone (T) might induce the synthesis of IGF-1 or IGF-binding protein (IGF-BP) which could affect cell proliferation. This was evaluated by determining the effect of T on thymidine incorporation, cell surface IGF-1 binding, and the production of IGF-1 and IGF-BP by cultured neonatal foreskin fibroblasts. Testosterone significantly increased thymidine incorporation and the production of IGF-1 and IGF-BP (p < 0.05 vs control). However, T significantly decreased the cell surface binding of IGF-1 (p < 0.0001 vs control). To determine whether or not the increase in IGF-1 production was important in mediating the effect of T on thymidine incorporation, cells were incubated with either anti-IGF-1 antibody (anti-IGF-1), anti-IGF-1-receptor antibody (IGF-1-R-Ab), or a non-specific control antibody (NS-Ab). Anti-IGF-1 significantly decreased thymidine incorporation in both control cultures and those containing T. In addition, IGF-1-R-Ab blocked the expected T dependent increase in thymidine incorporation, while NS-Ab had no effect. These in vitro observations suggest both T and IGF-1 affect neonatal foreskin fibroblasts in a complex relationship. In addition, these data suggest T might stimulate foreskin fibroblast proliferation, at least in part, by changing the balance in production and effects of IGF-1 and IGF-BP.