1. Voltage-clamp experiments have been performed on ovulated mouse eggs using the whole-cell recording technique. 2. Whole-cell recording offers improved signal-to-noise ratio and excellent stability over time. This allowed the study of the Ca2+ current of these eggs under physiological conditions (i.e. 1.7 mM-external Ca2+ and 37 degrees C). 3. In these conditions a negative shift of the reversal potential of the current (about -25 mV) and also of the activation and inactivation parameters (about -10 mV) compared with those recorded in 20 mM-external Ca2+ is found. 4. No significant diminution of the inward current was detected when external Na+ was substituted with impermeant cations, indicating no relevant participation of Na+ to the current in physiological conditions. 5. In a medium free of divalent cations a large inward current appeared, together with a large decrease in membrane resistance. 6. In Ca2+-free medium containing 1.2 mM-Mg2+ the inward current was largely suppressed, while an outward transient current appeared for depolarizations greater than +10 mV. 7. The 'outward surge current' previously described in this preparation appears to possess the same inactivation time constant and the same steady-state inactivation curve as the inward Ca2+ current. This suggests that the two currents flow through the same channels. 8. The time constant of inactivation was the same for both inward and outward currents and was independent of the current amplitude. These observations exclude a Ca2+-induced type of inactivation. 9. The channel which physiologically carries the Ca2+ current in mouse eggs belongs then to the class of Ca2+ channels that owe their selectivity to high-affinity Ca2+ binding sites.