Vagal control of heart rate (HR) is mediated by direct and indirect actions of acetylcholine (ACh). Direct action of ACh activates the muscarinic potassium (K_ACh) channels, whereas indirect action inhibits adenylyl cyclase. The role of the K_ACh channels in the overall picture of vagal HR control remains to be elucidated. We examined the role of the K_ACh channels in the transfer characteristics of the HR response to vagal stimulation. In nine anesthetized rabbits with sinoaortic denervation and vagotomy, the vagal nerve was stimulated using a binary white-noise signal (0-10 Hz) to examine the dynamic characteristic, and was stimulated in a stepwise manner (5, 10, 15, and 20 Hz every minute) to examine the static characteristic. The dynamic transfer function from vagal stimulation to HR approximated a first-order, low-pass filter with a lag time. Tertiapin, a selective K_ACh channel blocker (30 nmol kg^-1 iv) significantly decreased the dynamic gain from 5.0 ± 1.2 (mean ± SD) to 2.0 ± 0.6 beats min^-1 Hz^-1 (P<0.01) and the corner frequency from 0.25 ± 0.03 to 0.06 ± 0.01 Hz (P<0.01) without changing the lag time (0.37 ± 0.04 vs. 0.39 ± 0.05 sec). Moreover, tertiapin significantly attenuated the vagal stimulation-induced HR decrease by 46 ± 21, 58 ± 18, 65 ± 15 and 68 ± 11 % at stimulus frequencies of 5, 10, 15 and 20 Hz, respectively. We conclude that K_ACh channels contribute to a rapid HR change and to a larger decrease in the steady-state HR in response to more potent tonic vagal stimulation.