The activity of hypothalamic monoamines in response spontaneous feeding was investigated using the in vivo technique of brain microdialysis together with the instrumental recording of feeding pattern. The simultaneous variations of dopamine (DA), serotonin (5-HT), and their respective metabolites, DOPAC and 5-HIAA, were measured in the rostromedian hypothalamus, where the probe was located between the PVN and VMH. Throughout the experiment, the changes in DOPAC followed a mirror image of those in DA: DA regularly increased, reaching its zenith within the 15-min sample collected during the meal before returning to the same level as just before the meal. Following a premeal plateau, both 5-HT and 5-HIAA increased as soon as the beginning of feeding; 5-HT reached its zenith during the meal while 5-HIAA showed a more delayed and prolonged increase. When a new meal was initiated, 60 to 70 min later, a similar monoaminergic pattern was observed again. These data suggest that building up hunger is announced by an ascending slope of DA and setting up of satiation is concomitant with a descending slope of DA. Concerning serotonergic changes, the sharp 5-HT release during the meal would be a signal of satiation (transient preabsorptive fullness) while the longer-lasting increase in 5-HIAA, reflecting 5-HT synthesis, would be associated with satiety (more persistent postabsorptive state substituting satiation). These data partially confirm and extend previous pharmacological studies as well as the findings on deprivation-induced, imposed meals. They suggest a possible causal relation between monoaminergic changes and behavioral initiatives.