T he present investigation assessed the ability of atrial natriuretic factor (ANF) to affect hypothalamic neuronal excitability. Single neurons in the rostral septal-preoptic area of the female rat brain were recorded extracellularly and tested for responsiveness to iontophoretically applied and/or pressure-ejected ANF. Neurons responsive to ANF were detected in the lateral septal nucleus, the lateral paraolfactory area, the bed nucleus of the anterior commissure, and the medial preoptic area. The majority of neurons influenced by ANF (16 out of 18) exhibited a decrease in spontaneous firing rate during application of the peptide. In some cases, the inhibitory response outlasted the period of ANF application. When both iontophoretic and pressure ejection techniques were used to apply ANF to an individual neuron, the responses were similar. The results indicate that ANF is capable of modulating the membrane excitability of rat forebrain neurons and suggest that the peptide acts as a neuromodulator/neurotransmitter within the central nervous system.