Using double-labelling immunohistochemistry we have studied the localisation of leptin receptor proteins including both long and short forms and their possible presence in serotonergic (5-HT) and catecholaminergic neurons in the rat brain. Leptin receptor immunoreactivity was found to be widely distributed in the central nervous system including cortical areas, amygdala, several hypothalamic and thalamic nuclei, the raphe system, pontine nuclei, locus coeruleus, parabrachial nucleus, tractus solitarus and the medullary reticular formation. Serotonergic cell groups were identified by 5-HT immunocytochemistry and classified according to standard nomenclature. High degrees of co-existence of leptin receptor immunoreactivity with serotonin in the raphe system were observed in B1, B5, B6, B7, B8 and B9. In B3 and B2 less than 50% of the 5-HT cells colocalised leptin receptor immunoreactivity. Brainstem and diencephalic (catecholaminergic) neurons were identified by tyrosine hydroxylase immunocytochemistry and classified according to standard nomenclature. Within the periventricular hypothalamic dopaminergic nuclei A14 and A12, the metencephalic noradrenergic A6, A7, A2, A1, and the adrenergic C3, C2 and C1 cell groups, nearly all tyrosine hydroxylase-positive cells colocalised with leptin receptor immunoreactivity. In contrast, co-existence of tyrosine hydroxylase and leptin receptor immunoreactivities in the dopaminergic A13, A11, A10, A9 and A8 cell was practically non-existent. Thus leptin, the adipose tissue-derived ligand of the leptin receptor, may in some brain areas directly influence serotonergic, dopaminergic, adrenergic and noradrenergic inputs to the periventricular and medial hypothalamic nuclei.