Ivermectin has been shown to increase chloride conductances of invertebrate cells. On the muscle cells of the parasitic nematode Ascaris, ivermectin acts as both a GABA receptor antagonist and a chloride channel opener. In this study, ion-sensitive microelectrodes were used to investigate the effect of ivermectin on intracellular Cl- concentration of the somatic muscle bag cells of Ascaris suum. Incubation of muscle cells with ivermectin (10 microM in 1% dimethyl sulphoxide vehicle for 60 min) increased intracellular Cl- by 2.9 mM or 15% compared to controls (P < 0.01, n = 6).