The effects of each of the known platyhelminth neuropeptides were determined on muscle-strip preparations from the liver fluke, Fasciola hepatica. The activity of synthetic replicates of the C-terminal nonapeptide of neuropeptide F (NPF9, Moniezia expansa), and the FMRFamide-related peptides (FaRPs), GNFFRFamide, RYIRFamide, GYIRFamide and YIRFamide, were examined. Muscle-strip activity was recorded from 1 mm segments of muscle prepared from 28 to 32-day-old worms, using a photo-optic transducer system. None of the peptides (≤ 10 μM) altered baseline tension significantly; however, each of the peptides increased the amplitude and frequency of muscle contraction. The threshold for activity of each of the peptides examined was, respectively, 1 nM (RYIRFamide), 0·3 μM (GYIRFamide and YIRFamide), and 10 μM (GNFFRFamide and NPF9). All of the effects were reversible and repeatable, following wash-out. Muscle-strip integrity was tested following experimentation, using arecoline (10 μM) and high-K+ bathing medium (90 mM K+).