In a previous study, we established that leptin acts on chemosensitive intestinal vagal mechanoreceptors and that it's excitatory effects are blocked by the endogenous interleukin-1 receptor antagonist (Il- 1ra). To determine how interleukin-1 (Il-1& [beta]) is involved in the action of leptin, we studied the effects of this drug on the single vagal afferent activities of intestinal mechanoreceptors in anaesthetized cats. For this purpose, the activity of thirty-four intestinal vagal mechanoreceptors was recorded via glass microelectrodes implanted in the nodose ganglion. Il-1 (1 µg) administered into the artery irrigating the upper part of the intestine activated both the 16 leptin-activated units (type 1 units; P<0.01) and the 12 leptin-inhibited units (type 2 units; P<0.001), but had no effect on the 6 leptin-insensitive units. Cholecystokinin (CCK, 10 µg) induced an activatory response only in the two types of Il-1-sensitive units. When Il-1 was administered after CCK, its excitatory effects on type 1 units were enhanced, whereas the excitatory effects on type 2 units were abolished. Pre-treatment with Il-1ra (250 µg) blocked all the effects of Il-1& [beta] and the excitatory effects of leptin on type 1 units, whereas it enhanced the inhibitory effects of leptin on type 2 units. It can therefore be concluded that (i) leptin acts on intestinal vagal mechanoreceptors via Il-1 in the case of the type 1 units and independently of Il-1 in that of the type 2 units, and (ii) type 1 and type 2 units belong to two different populations of vagal afferents that transmit different information about ingestion or inflammation to the CNS, depending on the chemical environment.