Following horseradish peroxidase injections in the brainstem peribrachial (PB) area, massive retrograde labeling was found in the posterior hypothalamic region. Single-unit recordings posterior hypothalamic neurons with antidromically identified projections to the PB area revealed that these neurons have higher firing rates in waking than in slow-wave sleep and dissimilar discharge patterns as compared with intralaminar thalamic neurons. The results are discussed in the context of reciprocal hypothalamo-brainstem circuits.