Intracellular microelectrodes have been used in conjunction with ion substitution, and agonists and inhibitors of known transport processes to investigate the mechanisms whereby ions cross the basal and apical cell membranes of the Malpighian tubules of Locusta. Values for basal, apical and transepithelial potentials in 'Normal' saline were -71.6 ± 0.3 mV, -82.6 ± 0.8 mV and +5.7 ± 1.0 mV (lumen positive) respectively. Ion substitution experiments, involving Na(^+),K(^+) and C1(^-) in the bathing media, indicated that the basal membrane was more permeable to K(^+) than Na(^+) and C1(^-). Two different electrical responses to high [K(^+)](_o) saline (the Type A and Type B response) were noted and these probably reflect distinct physiological states of basal membrane permeability. Experiments with ouabain and vanadate suggested that whilst Na(^+)+K(^+) ATPase activity, which has been demonstrated in microsomal preparations, was not significantly electrogenic, asymmetric ionic distribution across the basal membrane was partly maintained by thisenzyme Furthermore, 3-H ouabain-binding studies indicated that Na(^+)+K(^+) exchange 'pump' turnover was adequate to account for substantial entry and Na^ exit across the basal membrane. The electrochemicalgradient across the apical membrane suggests that exit from the cell must involve an active process with CI following passively. Data from ion substitution experiments and treatment with furosemide and bumetanide suggest that CI entry across the basalmembrane may be via cotransport with Na^ and/or K^. However, the+ —differential electrical responses to Na(^+) free and C1(^-) free salines question the role of Na(^+) in this process. The effects of c AMP, Ca(^2+) substitution and various inhibitors on basal and apical membrane potentials, taken in conjunction with the results referred to above, are discussed and a hypothetical model proposed whereby changes in intracellular Ca(^2+) and c AMP effect control of ion movements across the two cell surfaces.