We have investigated the action of 9-amino-1, 2, 3, 4-tetrahydroacridine (THA) on the pacemaker current If in rabbit sino-atrial node myocytes and in canine Purkinje fibers. THA at concentrations in the range 3-300 microM blocked If in a voltage-independent manner, as revealed by measurements on the fully activated I/V relation for If. The dose/response relationship of the If maximal slope conductance (Gf) can be fitted by assuming a cooperative binding reaction where two THA molecules are required to block one If channel. Half-maximal block occurred at 18.2 microM in the sino-atrial node and 36.6 microM in Purkinje fibers. THA also affected the If kinetic properties. This was examined in the sino-atrial node where the current activation curve was shifted in the negative direction on the voltage axis (- 21 mV at 30 microM THA). The delayed rectifier current, IK, was also reduced by THA in sino-atrial node myocytes: at - 40 mV the IK fully activated value was decreased to 37% of its control value by 30 microM THA, with only a minor modification of the position of the activation curve at the same potential. Thus, although THA blocks If at a lower concentration than other known If-channel blockers [DiFrancesco (1982) J Physiol (Lond) 329:485-507], its action on the pacemaker current is not specific.