The identity of the potassium channel underlying the slow, apamin-insensitive component of the afterhyperpolarization current (sIAHP) remains unknown. We studied sIAHP in CA1 pyramidal neurons using simultaneous whole-cell recording, calcium fluorescence imaging, and flash photolysis of caged compounds. Intracellular calcium concentration ([Ca2+]i) peaked earlier and decayed more rapidly than sIAHP. Loading cells with low concentrations of the calcium chelator EGTA slowed the activation and decay of sIAHP. In the presence of EGTA, intracellular calcium decayed with two time constants. When [Ca2+]i was increased rapidly after photolysis of DM-Nitrophen, both apamin-sensitive and apamin-insensitive outward currents were activated. The apamin-sensitive current activated rapidly (<20 msec), whereas the apamin-insensitive current activated more slowly (180 msec). The apamin-insensitive current was reduced by application of serotonin and carbachol, confirming that it was caused by sIAHP channels. When [Ca2+]i was decreased rapidly via photolysis of diazo-2, the decay of sIAHP was similar to control (1. 7 sec). All results could be reproduced by a model potassium channel gated by calcium, suggesting that the channels underlying sIAHP have intrinsically slow kinetics because of their high affinity for calcium.