In has been supposed that the enhanced neural activity in hippocampus after conditioning is due to the modification of modulating inputs. The influence of serotonin application on the population spike amplitude in hippocampal slices of rats after conditioning was essentially less expressed than that in the slices prepared from the control rats. The after-effects of serotonin in the control slices were due to frequency facilitation in repetitive stimulation. Frequency facilitation was found in the hippocampal slices of conditioned rats before serotonin application. Serotonin did not influence this process. These findings are evidence in favour of an assumption that serotonin can be at least one of the causes of the frequency facilitation in hippocampus after conditioning.