The effects of a range of FMRFamide-related peptides (FaRPs) on isometric contractility were tested using isolated muscle strips from the liver fluke, F. hepatica. The neuropeptides tested were the molluscan FaRPs, FMRFamide and FLRFamide, the turbellarian FaRPs, RYIRFamide and GYIRFamide, the cestode peptides, NPF and GNFFRFamide, and the nematode FaRPs, AF-1 (KNEFIRFamide), AF-2 (KHEYLRFamide), AF-8 (KSAYMRFamide), and PF-4 (KPNFIRFamide). Dose-response experiments were undertaken at a concentration range of 5 nM–5 μM for all of the neuropeptides tested. FMRFamide and AF-8 caused statistically significant increases in the amplitude and frequency of contractions at concentrations of 0·5 μM and 5 μM. FLRFamide and AF-2 also caused significant increases in contraction frequency at concentrations of 0·5 μM and 5 μM, although a significant increase in amplitude of contraction was observed only at a concentration of 5 μM. GYIRFamide increased both amplitude and frequency significantly at concentrations of 50 nM, 0·5 μM and 5 μM. RYIRFamide significantly increased frequency of contractions at concentrations of 0·5 μM and 5 μM, but failed to have a significant effect on contraction amplitude. AF-1 at a concentration of 5 μM increased contraction amplitude, but failed to have an effect on frequency at any of the concentrations used. PF-4 caused a statistically significant increase in both the amplitude and frequency of contractions at a concentration of 5 μM. NPF and GNFFRFamide had no effect on the in vitro motility of F. hepatica over the range of concentrations tested. The results are discussed in the light of possible structure–activity relationships in the FaRPs tested.