The content of vasoactive intestinal peptide (VIP) was determined by radioimmunoassay in different brain regions of three fish species, namely salmon, trout and carp. The VIP extracted from salmon brain was shown by reverse-phase high-performance liquid chromatography to coelute with native porcine VIP. Highest VIP levels were measured in the salmon brain with a maximum of 112 pg/mg protein in the hypothalamus, followed by the telencephalon, olfactory bulb and optic tectum. In contrast, in the trout brain highest levels were found in the olfactory bulb (111 pg/mg protein). Lower levels were measured in the hypothalamus, telencephalon and cerebellum. The VIP content of the carp brain was considerably less, with highest values measured in the hypothalamus (12 pg/mg protein). The specificity in regional distribution further stresses the role of VIP in neurotransmission and indicates a function in neuroendocrine mechanisms and in the processing of olfactory stimuli.