The potency (muscle force-generated) of a number of long-chain RFamide neuropeptides was examined in mechanical experiments with the radular-retractor and radular-sac muscles of gastropods Buccinum undatum and Neptunea antiqua. Many of the heptapeptides, octapeptides and the decapeptide LMS were found to induce greater contraction than FMRFamide in both smooth muscles and in both species. RFamide neuropeptides interacted with the neurotransmitter acetylcholine in an additive way and RFamide-induced contractions were inhibited by the neuromodulator serotonin. Pre-treatment with a calcium-free saline completely abolished acetylcholine-induced responses but only partially inhibited RFamide responses in the muscles, suggesting that acetylcholine acts to cause influx of extracellular calcium for contraction. In contrast, RFamide neuropeptides may mobilise intracellular calcium to maintain sustained tonic force in calcium-free conditions. This suggests that an additional involvement of a fast calcium channel may be present in the RFamide responses, since loss of the usual superimposed twitch activity is observed. Force regulation in these muscles appears to result from a complex interaction of RFamide neuropeptides with the primary transmitter acetylcholine and the neuromodulator serotonin.