Brainstem visceral sensory and (nor)adrenergic neurons play crucial roles in modulating cardiovascular and respiratory functions. The origins and formation of these neurons are poorly understood. Here we show that these two classes of neurons are derived fromMash1-positive precursor cells, and can be prospectively identified by combinatorial expression of two homeobox genes,Rnx and Phox2 (Phox2a or Phox2b). It was previously shown that Rnx-deficient mice die from respiratory failure. Here we show that Rnx function is required for formation of first-order relay visceral sensory neurons in the brainstem. In addition, as in Phox2b-deficient mice, the development of most (nor)adrenergic centers is compromised inRnx mutants. We also provide genetic evidence to show that Rnx and Phox2 proteins may function independently to specify the (nor)adrenergic phenotype. Our studies reveal a surprising ontogenetic relationship between relay visceral sensory and (nor)adrenergic neurons, and suggest that it may be a common theme in the developing nervous system that the same set of transcriptional regulators is associated with formation of multiple components within a neuronal network.