Aminergic nuclear areas were identified in the brain of Acipenser ruthenus using formaldehyde-induced fluorescence (FIF). One group of scattered, green fluorescent cerebrospinal-fluid (CSF)-contacting neurons is located rostral to the commissura anterior. A second very prominent field of green fluorescent neurons is situated within the recessus preopticus, the recessus preopticus organ (RPO). Caudal to the latter there is a group of green and yellow fluorescent CSF-contacting neurons, the anterior part of the paraventricular organ (PVO). Further caudally, towards the dorsorostral entrance to the recessus lateralis the latter group is continuous with the main part of the PVO, which consists of large green and yellow, fluorescent CSF-contacting neurons in a stratified arrangement. The PVO is caudally continuous with small, green fluorescent CSF-contacting neurons situated within the roof of the recessus lateralis. The nucleus recessus posterior is made up of green and yellow fluorescent CSF-contacting neurons, which cover not only the walls of the recessus posteriores, but also line the lateral, dorsal and caudal aspects of the caudal hypothalamus. Small groups of green and yellow fluorescent CSF-contacting neurons can be found also in other areas of the hypothalamic ventricular walls, especially dorsal to the PVO and within the hypothalamic floor. Three large groups of aminergic neurons are present within the brain stem: One, yellow-fluorescent raphe-system and two green fluorescent groups. The floor of the canalis centralis is lined by green fluorescent CSF-contacting neurons. There is a very low density of aminergic terminals within the brain of Acipenser. A prominent fibre tract connects the PVO and the nucleus recessus posterior.(ABSTRACT TRUNCATED AT 250 WORDS)