1. The effect of synthetic tachykinin selective receptor agonists was studied on the growth of cultured human skin fibroblasts (HF). 2. Human fibroblasts were grown in serum-free conditions in the presence of natural tachykinins (substance P and neurokinin A) and of three synthetic agonists, [beta-Ala4, Sar9, Met(O2)11]-SP(4-11), [beta-Ala8]-NKA(4-10) and [MePhe7]-NKB selective for NK1-, NK2- and NK3-receptors respectively. Cell proliferation was measured by percentage increase in cell number and by [3H]-thymidine uptake following 48 h exposure to agents compared to baseline condition. 3. Neurokinin A (NKA) and substance P (SP) significantly increased cell proliferation the threshold concentrations being 10(-12) and 10(-11) M, respectively. Addition of thiorphan to culture conditions enhanced the effect of SP but not of NKA. 4. The selective NK1-receptor agonist produced a dose-dependent increase in cell proliferation as judged by total cell number and [3H]-thymidine uptake. No significant effect was observed with NK2- and NK3-receptor agonists. 5. These data indicate that the effect of SP on fibroblast proliferation is mediated by interaction with a NK1-receptor type and local metabolism can interfere with the full expression of this effect of SP on cell proliferation.