Atrial Natriuretic Peptide (ANP) and receptors for ANP are widely distributed in many tissues and cell types in vertebrates. ANP has been shown to be internalized into the cytoplasm in several cell types and thus it raises the possibility that it may act on intracellular receptors. Displacement experiments of [125I]-ANP binding to rat olfactory bulb mitochondrial fraction demonstrated the presence of high affinity (Kd < 10(-9)M) binding sites (Bmax, 112 fmol/mg protein) in this preparation. The addition of ANP (10(-8) M) to this mitochondrial preparation resulted in a 25% increase in TPP+ accumulation, signifying a striking hyperpolarization of the mitochondrial membrane. In contrast ANP did not increase TPP+ uptake to liver mitochondrial preparations. This direct effect of ANP on Olfactory bulb mitochondrial membrane potential may underly the known effects of this hormone on steroidogenesis.