Electrical properties of frog atrial myocardium under Ca-free conditions (30 min superfusion with 1 mM EGTA) were examined using a microelectrode and voltage clamp technique. Action potentials recorded from atrial trabeculae were extremely prolonged, up to more than 2 s. The resting potential was shifted towards more positive values (-60 +/- 9 mV (Ca-free); -75 +/- 4 mV (control); n = 6). Slow inward currents inactivated extremely slow showing time constants between 2 and 3 s. The voltage-dependence of the time constant of inactivation was found to be U-shaped in Ca-containing but bell-shaped in Ca-free solutions. After a 30 min Ca-free superfusion, the surfaces of the atrial trabeculae studied by scanning electron microscopy changes from being rough to extremely smooth, supposedly indicating a change in the surface coat. Using electron-microprobe analysis within a surface layer of about 0.2 micron thickness direct measurements of the Ca concentration showed a reduction under Ca-free conditions by more than 200-fold. Electrophysiological measurements, morphological findings and measurements of the Ca concentration in the surface region directly suggest the existence of a Ca-sensitive inactivation of slow channels in frog atrial myocardium.