Theta oscillations (5–10 Hz) can be observed during different behavior and physiological states such as exploration, learning or REM sleep and they are hypothesized to play important role in neuronal coding, learning and memory. Using sharp electrode intracellular recordings in rat hippocampal slices, we have now studied the mechanisms of spontaneous subthreshold theta oscillations evoked by steady depolarization beyond ­60 mV and slow depolarizing ramps. When the CA1 cells were depolarized, in the presence of fast synaptic transmission blockers (DNQX, APV and gabazine), they showed spontaneous subthreshold membrane potential oscillations (MPO) in the theta frequency range. Injection of an oscillating current with increasing frequency (1 to 15 Hz) evoked MPOs with a resonance peak in the theta frequency range (M-resonance). The spontaneous MPOs and resonance were suppressed by INaP blocker TTX (1 M, n=8). The M-channel blocker XE991 (10 M) blocked M-resonance and facilitated firing that replaced the spontaneous MPOs, whereas a lower dose (2 M) slowed down the MPOs (n=5). The M-channel opener retigabine (10 M) enhanced the spontaneous subthreshold theta oscillations and M-resonance (n=7) as expected for M-current dependent processes. In contrast, when the cell was hyperpolarized (­70 to ­80 mV) there were no spontaneous theta oscillations and the theta resonance at those potentials was not blocked by XE991 or TTX, but was suppressed by the h-channel blocker ZD7288, which also blocked the sag in response to hyperpolarizing pulses (n=6). These results indicate that CA1 hippocampal pyramidal cells, when depolarized to potentials just subthreshold for spike generation, generate spontaneous theta oscillations caused by M- and NaP-currents, but not HCN/h currents.