1. Whole-cell voltage clamp techniques were used to examine acetylcholine (ACh)-elicited currents in differentiated cells of the rat phaeochromocytoma cell line, PC12. 2. In the absence of intracellular Mg2+, the whole-cell current-voltage relationship for the ACh-elicited current displayed inward rectification which was reduced in part by the presence of 5 mM internal adenosine 5'-triphosphate (ATP). 3. The reduction in the rectification attributed to ATP developed over the first 15-20 min of whole-cell recording. Similar results were obtained with a non-hydrolysable ATP analogue, adenosine-5'-O-3-thiotriphosphate (ATP gamma S), or cytosine 5'-triphosphate (CTP) in the internal solution, but not with adenosine 5'-diphosphate (ADP) or pyrophosphate. 4. The magnitude of the ACh-elicited current was also dependent on recording time and the composition of the internal pipette solution. The magnitude of the peak ACh-elicited current increased over time when the cell was internally perfused with the control solution or a pipette solution containing pyrophosphate, ATP gamma S, or ADP. The largest sustained increases in ACh-elicited current were observed in the presence of internal pyrophosphate or ATP gamma S. In contrast, with internal ATP or CTP, the whole-cell current initially increased, then steadily decreased with recording time. 5. The desensitization rate of the ACh-elicited current increased with recording time irrespective of the composition of the intracellular solution. 6. The actions of the compounds tested make it likely that the changes in the whole-cell current-voltage relationship, peak current, and desensitization are produced by separate mechanisms. The mechanisms underlying these changes are unknown, but the ability of the compounds to chelate divalent cations is unlikely to be the explanation. Other unlikely explanations include phosphorylation of the ACh receptor or regulation by GTP-binding proteins.