Unit activity of the hippocampus, the centrum-medianum-parafascicularis and medialis dorsalis nuclei of the thalamus, was recorded in chronic rats during a classical conditioning; neocortical electroencephalographic and somatic responses were also recorded. Conditioned unit responses of the different groups of neurons were compared according to the precocity of their appearance, their stability, their latency and the differentiation between the positive (reinforced) and the negative (non-reinforced) conditioned stimuli. Conditioned unit responses of type I hippocampal neurons (probably pyramids) and of neurons located in the centrum-medianum-parafascicularis nucleus did not differentiate between the positive and negative conditioned stimuli; they progressed rapidly, then declined and disappeared. They were contemporary with an initial conditioning stage which was characterized by undifferentiated arousal responses (desynchrony of electroencephalographic activity) to both conditioned stimuli. Conditioned unit responses of type II hippocampal neurons (probably granule cells or interneurons of the fascia dentata) and of neurons located in the medialis dorsalis nucleus progressed slowly and differentiated between the conditioned stimuli during a late conditioning stage which was characterized by the regression of arousal responses and the differentiation of somatic responses. These data strongly suggest that two neuronal systems, each having a different role, are involved in classical conditioning in the rat.