The elementary conductance of the slow inward current channel in the rabbit sinoatrial node was measured by analysis of the current fluctuations. The preparations were voltage-clamped to -30 mV, where d infinity and f infinity of the slow inward current (Isi) intersect. In the presence of Ba, which increases Isi and decreases outward currents, a small steady-state component of Isi could be detected. The fluctuations of the current in 10 mM Ba were smaller due to the block of the outward channels. They were further reduced after the inhibition of Isi by the Ca channel blocker D600. The spectral power density distribution of the current fluctuations originating from Isi could be fitted at frequencies less than 30 Hz with a single Lorentzian which was attributed to the inactivation process. The corner frequency was 5.28 +/- 1.16 Hz (n = 10), corresponding to an average open time of the single channel of about 30 msec at -30 mV. The single channel conductance was determined to 6.50 +/- 3.15 pS (S.D., n = 10).