This study was conducted to describe the cardiovascular responses to intra-arterial injections of serotonin in the Antarctic fish Pagothenia borchgrevinki and to elucidate the underlying mechanisms, Immunohistochemistry was used to localise serotonin-containing cells within the gills, Simultaneous and continuous recordings of ventral and dorsal aortic blood pressure, heart rate and ventral aortic blood flow (cardiac output) were made using standard cannulation procedures in combination with Doppler flow measurement, An extracorporeal loop with an in-line oxygen electrode allowed continuous measurements of arterial oxygen pressure Pa-O2. Pre-branchial injection of serotonin (5-hydroxytryptamine, 5-HT) or the 5-HT2 receptor agonist alpha-methylserotonin increased the branchial vascular resistance and ventral aortic pressure, while the 5-HT1 receptor agonist piperazine was without effect, The branchial vasoconstriction produced by serotonin injection was completely blocked by the 5-HT1/5-HT2 receptor antagonist methysergide and the branchial vasoconstriction produced by alpha-methylserotonin injection was completely blocked by the specific 5-HT2 receptor antagonist LY53857, The results suggest that the 5-HT2 receptor alone mediates the branchial vasoconstriction. Serotonin also mediated a methysergide-sensitive reduction in Pa-O2, the reduction being greatest when the pre-injection Pa-O2 value was high, 5-HT-immunoreactive cells and nerve fibres were present within the gill tissues, All the 5-HT-immunoreactive cells were located on the efferent side of the filaments, but 5-HT-immunoreactive nerve fibres were found lining both of the branchial arteries, Our findings demonstrate a potential serotonergic control system for the gills in Pagothenia borchgrevinki. In contrast to its effects on the branchial vasculature, serotonin produced a methysergide-insensitive decrease in the systemic vascular resistance, However, neither the specific 5-HT1 nor 5-HT2 receptor agonists produced a decrease in the resistance of the systemic vasculature, The nature of the serotonergic receptor(s) inducing vasodilation in teleost fish is uncertain.