INTRODUCTION: Neuronal synchronization depends on many factors including HCN channel action. They are voltage-gated ion channels that mediate an inward cationic current dependent on hyperpolarization. There is sparse evidence for their contribution to neuronal plasticity, learning and memory, epilepsy, or Alzheimer’s disease. HCN channels can be found in the hippocampus (HPC) and are thought to be involved in neuronal synchronization through initiating membrane potential oscillations which are necessary for the appearance of field theta oscillations. Hippocampal theta rhythm is known to be involved in memory formation, spatial navigation, sensorimotor integration, movement initiation, and others. So far it is established that HPC theta generation is a result of a fine balance between the cholinergic and GABAergic system activation, which triggers the synchronous action of theta-related neurons. However, the involvement of HCN channels in this process is still mostly unknown. AIM(S): The aim of this study was to investigate the role of HCN channel activation in the process of theta generation. METHOD(S): Three experimental models were used: in vivo anesthetized rats, in vitro acute HPC slices, and HPC patch clamp whole cell method. Field and single neuron recordings were made from the HPC after perfusion with a non-specific HCN channels agonist – lamotrigine (LTG). RESULTS: When LTG was applied it produced mixed results. In particular, it blocked theta rhythm in vitro but significantly enhanced it in vivo. Patch clamp results have shown that LTG reduced the frequency of spontaneous inhibitory postsynaptic currents but also decreased the excitability and membrane resistance of CA1 neurons. Also, LTG reduced membrane potential theta resonance in most CA1 cells. CONCLUSIONS: HCN channels activation was shown to have an impact on the process of theta rhythm generation in the HPC. Current results are discussed. FINANCIAL SUPPORT: Supported by National Science Centre, grant no. 2017/26/D/NZ4/00159.