Squid giant axons were injected with aequorin and then treated with seawater containing 50 mM Ca and 100-465 mM K+. Measurements of light production suggested a phasic entry of Ca as well as an enhanced steady-state aequorin glow. After a test K+ depolarization, the aequorin-injected axon was stimulated for 30 min in Li seawater that was Ca-free, a procedure known to reduce [Na]i to about one-half the normal concentration. Reapplication of the elevated K+ test solution now showed that the Ca entry was virtually abolished by this stimulation in Li. A subsequent stimulation of the axon in Na seawater for 30 min resulted in recovery of the response to depolarization by high K+ noted in a normal fresh axon. In axons first tested for a high K+ response and then stimulated in Na seawater for 30 min (where [Na]i increases approximately 30%), there was approximately eight fold enhancement in this response to a test polarization. Axons depolarized with 465 mM K seawater in the absence of external Ca for several minutes were still capable of producing a large phasic entry of Ca when [Ca]0 was made 50 mM, which suggests that it is Ca entry itself rather than membrane depolarization that produced inactivation. Responses to stimulation at 60 pulses/s in Na seawater containing 50 mM Ca are at best only 5% of those measured with high K solutions. The response to repetitive stimulation is not measurable if [Ca]o is made 1 mM, whereas the response to steady depolarization is scarcely affected.