The biophysical properties of the nicotinic acetylcholine receptor (AChR) are known to be modulated by some steroid hormones; their precise mechanism of action is, however, unclear. A possible direct effect of the glucocorticoid corticosterone (COR) on AChRs of mouse C2C12 myotubes was studied in the cell-attached patch-clamp configuration. When added to pipette solution containing acetylcholine, COR had no effect on the single channel conductance, but it reduced the longest time constant of both open time and burst duration histograms by 55% and 65%, respectively. COR also increased nearly by 10-fold the middle time constant of the closed time histogram. COR coupled to a hydrophilic molecule such as bovine serum albumin, however, affected only the closed time distribution. These results suggest that existence of specific recognition sites for COR on the surface of the cell membrane and/or the AChR protein.