1. Whole-cell and patch clamp techniques have been applied to cells isolated from the rabbit sino-atrial (S-A) node to study the ionic mechanism(s) of adenosine-induced slowing of cardiac pacemaker activity. 2. Viable spontaneously active cells were isolated from the central region of the S-A node of the rabbit heart by an enzymatic dispersion procedure similar to that reported by Giles & van Ginneken (1985) and van Ginneken & Giles (1988). In these spontaneously beating cells application of adenosine caused a dose-dependent slowing accompanied by a small hyperpolarization of the maximum diastolic potential. Relatively high doses of adenosine (greater than 20 microM) caused complete arrest, associated with a hyperpolarization of 12-15 mV. 3. In corresponding whole-cell voltage clamp experiments adenosine activated a time-independent potassium current, IK(ADO), which at -50 mV is approximately 30 pA in normal Tyrode solution and 50 pA in high [K+]o (20 mM) Tyrode solution. This current is similar to the one identified previously in guinea-pig atrium (Belardinelli & Isenberg, 1983a; Kurachi, Nakajima & Sugimoto, 1986). 4. Patch clamp recordings of the single-channel events underlying IK(ADO) showed that they have a conductance of approximately 25.0 +/- 1.9 pS. The whole-cell or macroscopic current, IK(ADO), and the adenosine-induced single-channel events exhibit strong inward-going rectification. 5. Adenosine in doses (10 microM) which significantly activate IK(ADO) failed to produce any measurable effect on the calcium current, ICa, in these isolated cardiac pacemaker cells. However, after ICa has been enhanced by the addition of isoprenaline, adenosine (1-10 microM) caused a significant inhibition: it reduced ICa back to approximately the control levels. 6. A similar 'indirect' effect of adenosine was observed on If, the slow time- and voltage-dependent inward current which is activated by hyperpolarizing these S-A node cells. Adenosine (10(-5) M) failed to influence the control or basal If; however, after If was enhanced by isoprenaline, adenosine markedly inhibited it. 7. These results provide explanations for both the direct and the indirect effects of adenosine in mammalian cardiac pacemaker tissue: activation of IK(ADO), and of a time-independent background potassium current and inhibition of ICa and If, respectively. Since it is known that there is significant adrenergic tone in the mammalian S-A node both the indirect and the direct effects of adenosine may be of physiological importance.