Polybrominated diphenyl ethers (PBDEs), used as additive flame-retardants, are increasing in the environment and are present in human mother's milk, newborns and toddlers. We reported earlier that several PBDEs, highly brominated PBDEs, caused developmental neurotoxic effects in mice, manifested as persistent aberrations in spontaneous behaviour, habituation capability, learning and memory, and changes in the cholinergic system. The present study was undertaken to explore the dose-response effects of PBDE 209 on spontaneous behaviour, habituation and its effects on the murine cholinergic system. Neonatal male NMRI mice were given 1.4, 2.3, 14 or 21micromol PBDE 209/kg body weight, when 3 days old. The agent was administered as a single oral dose via a metal gastric tube. Spontaneous behaviour and response to the cholinergic agonist nicotine were observed in adult mice at 2 and 4 months of age. Mice were also observed for anxiety-like behaviour in an elevated plus-maze. Adult mice, 2 and 4 months old, showed a dose-response related change in spontaneous behaviour, viz. were hyperactive and showed reduced or lack of habituation, effects that worsen with age. At the adult age of 4 months the susceptibility of the cholinergic system was also affected in a dose-response related manner, viz. reduced and/or hypoactive response to nicotine. This shows that PBDE 209 can be as potent as the lower brominated PBDEs in causing developmental neurotoxic defects.