Fine wire microelectrodes were implanted for diagnostic purposes in 17 patients suffering from psychomotor epilepsy. Single- and multiunit activity during waking and natural nocturnal slow wave sleep and REM sleep was recorded in the hippocampus (n = 42), hippocampal gyrus (n = 53), and amygdala (n = 32). The firing rates of hippocampal gyrus units usually decreased during slow wave sleep and then increased to levels equal to or above waking during REM. In contrast, the firing rates of hippocampal neurons generally increased during slow wave sleep and fell to very low levels during REM. The amygdala presented a more mixed pattern. Since the projection from the hippocampal gyrus to hippocampus is excitatory, their opposite patterns during sleep suggest that the tonic firing patterns of HC neurons may be mainly the result of subcortical afferents.