INTRACELLULAR recordings in an in vitro neocortical slice preparation from immature rats were used to investigate the Ca2+ source for slow afterhyperpolarization (sAHP) generation in pyramidal neurons that exhibit complete spike frequency adaptation (CA neurons). In pyramidal neurons that maintain repetitive firing for long periods of time (RF neurons), N-, P- and Q-type Ca2+ channels supply Ca2+ for sAHP generation. In CA neurons, the sAHP was reduced by only 50% by the combination of antagonists for these Ca2+ channel types and L-type channels. Ryanodine and dantrolene, blockers of Ca2+ −induced Ca2+ release, reduced the sAHP by ∼45% in CA neurons, but caused no reduction of the sAHP in RF neurons. Dantrolene application caused CA neurons to fire throughout a 1 s suprathreshold current injection (as do RF neurons).