The three–spined stickleback Gasterosteus aculeatus is an intermediate host of the tapeworm Schistocephalus solidus. Changes in predator avoidance, foraging and shoaling behaviour have been reported in sticklebacks infested with S. solidus, but the mechanisms underlying parasite–induced behavioural changes are not understood. Monoamine neurotransmitters are involved in the control of behaviour and central monoaminergic systems are sensitive to various stressors. Thus, the behavioural effects of S. solidus infestation might be a reflection of changes in brain monoaminergic activity in the stickleback host. The concentrations of 5–hydroxytryptamine (5–HT), dopamine (DA), norepinephrine (NE) and their metabolites 5–hydroxy–indoleacetic acid (5–HIAA), homovanilic acid (HVA) and 3–methoxy–4–hydroxyphenylglycol (MHPG) were measured in the telencephalons, hypothalami and brainstems of parasitized and non–parasitized female sticklebacks held in the laboratory. The ratios of 5–HIAA:5–HT were significantly elevated in both the hypothalami and brainstems of infected sticklebacks. The concentrations of 5–HTand NE were significantly reduced in the telencephalons of infected fish as compared with controls, but there was no elevation of metabolite concentrations. The results are consistent with chronic stress in infected fish, but may also reflect other alterations of neuroendocrine status resulting from parasite infection.