The resting membrane potential of Ascaris muscle bag cells is quite low at around -30mV. The most important ion in resting membrane potential determination is Cl⁻, giving a 13mV change/10 fold change in extracellular Cl⁻ concentration. Cl⁻ is also the ion that mediates the GABA inhibitory response of these cells, and is actively distributed, with E_C1 typically 20mV more negative than Em. Due to the apparent importance of Cl⁻ in the physiology of Ascaris muscle bag cells, these studies were conducted to investigate the mechanisms involved in Cl⁻ balance, with a view to possible exploitation of these mechanisms as anthelmintic target sites. The anion selectivity of the GABA-Cl channel was investigated, selectivity was determined using the reversal potential of the GABA event (E_GABA). The cut-off point for permeation through these channels was found to be between the relative hydrated sizes of ClO₃^- and BrO₃^-, indicating a diameter for the selectivity filter of between 0.29 and 0.33nm. The selectivity sequence of small anions was consistent with an Eisenman sequence 1, indicating that the selectivity determining region of the channel contained a low field strength site. Both the above observations are typical for ligand gated Cl⁻ channels. Ion sensitive microelectrodes (ISMs) incorporating the ion exchanger Corning 477913, were used to measure intracellular Cl⁻ concentration in the muscle bag cells. Cl⁻ concentration was found to be about 18mM, and for all cells tested was lower than predicted from a passive distribution, indicating the presence of a Cl⁻ extrusion process. Intracellular Cl⁻ was found not to fall as expected when extracellular Cl⁻ was replaced, and simultaneous E_GABA determinations confirmed the presence of interfering ions in the bag cells. These overestimated the measured apparent Cl⁻ in 9mM extracellular Cl⁻ by 8mM. Incubation of Ascaris sections with the anthelmintic ivermectin (IVM, 10μM) elicited an approximate 2.9mM (15%) increase in intracellular Cl⁻ concentration compared to control sections. Incubation of sections with IVM also increased the degree of hyperpolarization when gluconate was replaced by Cl⁻ and NO₃⁻, indicating an increase in the anion conductance of the muscle bag cell membrane.