Neuropeptide Y receptors in the medulla oblongata participate in central cardiovascular control. The neuropeptide Y1 receptor subtype gene and amino acid sequence have been identified by molecular cloning studies. In this study, a C-terminal peptide representing amino acids 355-382 of the neuropeptide Y1 receptor was synthesized and cross-linked to thyroglobulin to produce an antibody against a partial sequence of the neuropeptide Y1 receptor, used to localize neuropeptide Y1 receptor-like immunoreactivity in the catecholaminergic neurons of the medulla oblongata. The double colour immunofluorescence technique with a polyclonal antibody against the neuropeptide Y1 receptor and a monoclonal antibody against tyrosine hydroxylase revealed that in the rat medulla oblongata, a weak (the C3 cell group) to moderately intense (the A1, A2, C1 and C2 cell groups), diffuse cytoplasmic neuropeptide Y1 receptor-like immunoreactivity was distributed primarily in the noradrenergic and adrenergic cell bodies and occasionally seen in the noradrenergic and adrenergic cell processes. Almost all tyrosine hydroxylase-like immunoreactive cell bodies in the A1, A2, C1, C2 and C3 cell groups showed neuropeptide Y1 receptor-like immunoreactivity. The neuropeptide Y1 receptor-like immunoreactivity in the A2 cell group was somewhat stronger. The present findings show localization of specific neuropeptide Y1 receptor-like immunoreactivity in the vast majority of the noradrenergic and adrenergic cell bodies of the A1, A2, C1, C2 and C3 cell groups, which are putative cardiovascular regions. The results support the view that neuropeptide Y1 receptors in the medulla oblongata are involved in central cardiovascular control and may coexist with another important receptor, the alpha 2A-adrenoceptor, also involved in central, cardiovascular regulation, since the alpha 2A-adrenoceptor-like immunoreactivity has been shown to exist in almost all noradrenergic and adrenergic cell bodies in the brainstem. In conclusion, centrally administered neuropeptide Y may act in part via neuropeptide Y1 receptors located on the soma and dendrites of noradrenergic and adrenergic neurons, where it may interact with alpha 2-adrenoceptors at least in the noradrenergic A2 neurons. This noradrenaline system may be involved in at least part of the vasodepressor actions of neuropeptide Y, noradrenaline and adrenaline in the nucleus tractus solitarii in view of the present findings.