The effects of neurotensin (NT) on 41 spontaneously active neurons in the rat hypothalamic arcuate nucleus (ARC) were determined using an in vitro brain slice technique. Addition of NT (1 microM to 10 nM) to the perifusing medium allowed the action of known concentrations of NT to be examined. Concentrations of peptide greater than 10 nM evoked excessive depolarization of sensitive neurons resulting in long periods of postexcitatory inhibition. At 10 nM, NT evoked reversible and repeatable excitatory (20%) and inhibitory (25%) responses. When synaptic activity was blocked by lowering the Ca2+ and raising the Mg2+ concentration of the medium only excitatory responses (25% of units tested) were observed. These results demonstrate a potent excitatory action of NT within the hypothalamic ARC at the single cell level.