Methods employing suction electrodes to measure the small inward currents associated with the exchange of internal Ca2+ for external Na+ in salamander rod outer segments are described. 2. The ratio of the integral of the exchange current to the integral of the Ca2+ current during the loading period averaged 0.37, which is consistent with 1 Ca2+ ion exchanging with 2.7 Na+ ions, in approximate agreement with Yau & Nakatani (1984b). 3. The transient pumping current observed when external Na+ was restored after a few seconds in isotonic Ca2+ with IBMX (3-isobutyl-1-methylxanthine) consisted of a phase with current at a constant saturated level followed by a phase in which current declined along a characteristic S-shaped curve that was much steeper than expected from the Michaelis equation. 4. The relation between Ca2+ load and pumping current was also steeper than a Michaelis relation. 5. In Ringer solution at 20 degrees C the saturated exchange current was about 20 pA and the value of charge at which the current was half-saturated was 1-5 pC corresponding to 6-30 X 10(6) Ca2+ ions per rod outer segment. 6. The Ca2+ exchange current after small loads declined along the same curve as that determined with medium loads but fell more slowly after large loads. 7. The exchange current at the beginning of the plateau of a strong flash response usually declined along the curve determined with small or medium Ca2+ loads. 8. There was evidence that the exchange current at the tip of the outer segment remained saturated for longer than at the base. 9. The time to pump out Ca2+ through the Na+-Ca2+ exchange system is largely responsible for the delay in the recovery of the light-sensitive current after a Ca2+ load. 10. A theoretical analysis of some of the observations in this and the succeeding paper is based on assumptions about the binding of Ca2+ by exchange sites and by cytoplasmic Ca2+ buffers.
