Relatively little is known about subthreshold theta resonance and after-hyperpolarization (AHP) mechanisms in CA3 pyramidal neurons – processes that may be important for hippocampal theta oscillatons, excitability and plasticity. Using whole cell patch clamp recordings in rat hippocampal slices, after blocking fast synaptic transmission, we found that CA3 pyramidal cells showed subthreshold (beyond ­60 mV) membrane potential oscillations (MPOs, n=24) at 4–9 Hz, and typical M-resonance (Hu & al. 2002). Thus, injection of an oscillating current with increasing frequency (ZAP) evoked MPOs with a resonance peak in the theta range. The spontaneous MPOs and resonance were suppressed by TTX (n=7). The M-channel blocker XE 991 (10 M) partially suppressed the spontaneous MPOs and reduced their frequency, and the M-resonance and medium-duration AHP (mAHP) were blocked (n=7). Carbachol (30 M) produced a similar suppression (n=9), whereas 1 M atropine caused recovery of MPOs, as expected for muscarinic suppression of M-current-dependent processes. In contrast, H-resonance, which is mediated by HCN/h current, was observed at hyperpolarized membrane potentials (­70 to ­80 mV) and was not blocked by carbachol or TTX, but was suppressed by the h-channel blocker ZD7288 (10 M, n=5). These results indicate that CA3 hippocampal pyramidal cells are equipped with voltage-dependent dual mechanisms of theta resonance: M- and H-resonance, caused by M-, NaP- and h-currents, and in addition M-current-dependent mAHP and spike frequency adaptation mechanisms, similar to those found in CA1 cells. These mechanisms are likely to contribute to hippocampal theta oscillations and excitability control. [Supported by NFR/ SFF/ STORFORSK/ FUGE]