OBJECTIVE Type II pyrethroids are a group of insecticides largely used in agriculture and public health. The nervous system is the main target for pyrethroids in insects and mammals. One notable form of toxicity associated with over exposure has been a facial cutaneous paraesthesia and irritation-related respiration symptoms including behavioural excitation mainly observed in workers spraying pyrethroids or in occupational settings. In acutely exposed rats, type II pyrethroids produce a severe syndrome characterized by salivation and choreoathetosis. Because many of the acute functional effects of type II pyrethoids can be associated with the neurotoxic effect on 5-hydroxytryptamine (5-HT) neurones, the objective of the present study was to examine whether deltamethrin, cyfluthrin and lambda-cyhalothrin administration results in changes of 5-HT content in rat brain. Characterizing this target will help us to better understand the toxicological effects of type II pyrethroids. METHODS Rats were injected with either corn oil or pyrethroids (deltamethrin, 20 mg/kg per day, i.p., for 6 days; cyfluthrin, 14 mg/kg per day, i.p., for 6 days; lambda-cyhalothrin, 8 mg/kg per day, i.p., for 6 days). The frontal cortex, hippocampus, midbrain and striatum were removed at 24 hours post treatment and were analysed for content of 5-HT and 5-HIAA using a HPLC method with electrochemical detection. RESULTS A serotonin depleting effect was produced by these type II pyrethroids. The concentration of 5-HT and its metabolite 5-HIAA decreased in the brain regions from pyrethroid treated animals. Pyrethroids accelerated the turnover of 5-HT in midbrain and striatum areas. It is concluded that pyrethroids affect serotonin neurotransmission.