The electrophysiological properties of the bag region of the somatic muscle cells of Ascaris suum and Ascaridia galli were studied using intracellular techniques. For Ascaris muscle cells, the mean resting membrane potentials at 20 and 37°C were −29·9 and −33·8 mV respectively, and the average input conductance was 2·12 μS. For the muscle cells of A. galli similar values were obtained. For example, the mean input conductance of these cells was 2·84 μS at 20°C. Healthy Ascaris muscle cells at near physiological temperatures show both spontaneous depolarizing and hyperpolarizing activity and, in cells close to the nerve cords, rhythmic large amplitude (approximately 30 mV) action potentials are observed. Such action potentials, which are very sensitive to temperature variations, originate in the muscle cells. In contrast the muscle cells of Ascaridia are quiescent. The rhythmic action potentials of Ascaris are resistant to tetrodotoxin (TTX) (≤ 10−6 M), verapamil (10−4 M) and cinnarizine (10−4 M), but are blocked irreversibly by 22, 23 dihydroavermectin B1a (10−7 to 5 × 10−6 M). GABA, and the GABAA receptor agonists, muscimol and isoguvacine, hyperpolarize and increase the input conductance of both Ascaris and Ascaridia muscle cells. The antagonists+bicuculline and picrotoxin were not effective in modulating the spontaneous hyper polarizations of Ascaris muscle cells, and picrotoxin (10−4 M) was not effective in altering the response to GABA (5 × 10−6 M). The significance of the results is discussed briefly.