Acetylcholine (ACh) activates the inwardly rectifying muscarinic K⁺ channel in rat atrial cells via pertussis toxin (PTX)-sensitive G-protein (G_k) coupled with the muscarinic receptor (mAChR). Although this K⁺ (K_ACh) channel function has reported to be modulated by the phosphorylation process, a kinase and phosphatase involved in these processes are still unclear. Since either PKA or PKC was not effective on this ATP-modulation, the present study examined the possible involvement of the protein tyrosine kinase (PTK) and protein tyrosine phosphatase (PTP) in the function of the K_ACh Channel. In the inside-out (I/O) patch preparation excised from the adult rat atrial cell, when activated by 10 μM ACh in the pipette and 100 μM GTP in the bath, the mean open time (τ_o) and the channel activity (K_ACh) was 1.13 ms (n=5) and 0.19 (n=6), respectively. Following the application of 1 mM ATP into the bath, τ_o increased by 34% (1.54 ms, n=5) and K_ACh by 66% (0.28, n=6). Channel function elevated by ATP was lasted after washout of ATP. However, this ATP-induced increase in the K_ACh channel function did not occur in pretreated cells with genistein (50 ~ 100 μM), a selective PTK inhibitor, but occurred in pretreated cells with equimolar daidzein, a negative control of the genistein. On the contrary, PTP which acts on tyrosine residue conversely reversed both ATP-induced increased τ_o by 32% (1.20 ms, n=3) and K_ACh by 41% (0.15, n=3), respectively. Taken together, these results suggest that K_ACh channel may, at least partly, be regulated by the tyrosyl phosphorylation, although it is unclear where this process exerts on the muscarinic signal transduction pathway comprising the mAChR-G_k-the K_ACh channel.