This study investigated a possible participation of the centrifugal serotonergic (5-HT) innervation of the olfactory bulb (OB) in the control of the multiunit mitral cell responsiveness in behaving rats. A bilateral injection of 5,6-dihydroxytryptamine solution (5,6-DHT) into the OB induced in this structure an average 70% reduction in the level of endogenous serotonin. Control and treated animals were habituated to eat a single 2 h daily meal. Recording sessions were performed either after a 22 h fasting period or just after the meal. The summated multiunit mitral cell activity and vigilance state parameters were recorded in the absence of and during repeated stimulation by puffs of food odour and isoamyl acetate. The results showed that the 5,6-DHT treatment did not modify significantly previously established centrifugal controls exerted at the OB level: (1) the selective enhancement of the mitral cell responsiveness to food odour occurring in waking food-deprived animals; (2) the inhibition of the olfactory input during slow wave sleep. However, the considered 5-HT projections could mediate some inhibitory influence on mitral cell activity during wakefulness only. Finally, the 5-HT innervation of the OB did not seem to be involved in basic mechanisms of olfactory recognition.