AF3 (AVPGVLRFamide) and AF4 (GDVPGVLRFamide) are endogenous RFamide-like peptides isolated from the parasitic nematode Ascaris suum. Here the actions of these peptides on the somatic musculature of Ascaris have been investigated and compared to the action of acetylcholine (ACh), the excitatory transmitter at the neuromuscular junction. ACh, AF3 and AF4 contracted muscle with EC50s of 13±1 μM, 24±6 nM and 37±2 nM, respectively (n=6). The muscle cells were depolarized by ACh (3 μM; 5·2±0·4 mV, n=42), AF3 (1 μM; 2·6±0·3 mV, n=19) and AF4 (1 μM; 3·3±0·4 mV, n=19). EC50s were 681±329 nM (AF3) and 901±229 nM (AF4), but an estimate could not be made for ACh due to muscle contraction at concentrations greater than 10 μM. The depolarization to 3 μM ACh was abolished by the nicotinic receptor antagonist mecamylamine (10 μM; n=5) but the responses to the peptides were not (111±7% and 108±17% with respect to control; n=5). The depolarization elicited by ACh was reduced to a greater extent by a 50% reduction in extracellular Na+ concentration than the response to AF3 and AF4 (P<0·02). Cobalt was more effective at blocking the AF3 and AF4 depolarizations than those to ACh. These observations suggest that AF3 and AF4 contract Ascaris muscle without an action at the Ascaris nicotinic receptor. Furthermore, the ionic mechanism through which AF3 and AF4 depolarize Ascaris muscle is different from that for ACh. ACh, AF3 and AF4 were also found to contract Ascaridia galli somatic muscle with EC50s of 13±3 μM, 721±236 nM and 371±177 nM, respectively (n=7). The muscle cells were depolarized by ACh (EC50=14±5 μM, n=5), AF3 (EC50=5±3 μM, n=4) and AF4 (EC50=10±5 μM, n=4). Therefore the response to these peptides is not unique to Ascaris and they may subserve a functional role in the motor nervous system of parasitic nematodes.