Previous research in the rat has demonstrated that neonatal exposure to bacterial endotoxin alters the level of anxiety-like behaviour displayed in adulthood. Currently, however, little is known about the emergence and development of this type of behaviour. Given the ability of neonatal endotoxin exposure to alter neural substrates involved in regulating anxiety, we tested the hypothesis that it may also alter the developmental trajectory of anxiety-like behaviour in the rat. Male Fischer 344 neonatal rats were treated with endotoxin (0.05 mg/kg lipopolysaccharide from Salmonella enteriditis) or vehicle on postnatal days 3 and 5. Age related changes in anxiety-like behaviour were subsequently investigated using the elevated plus maze apparatus at three developmental time points; adolescence (43 days), adulthood (80 days) and senescence (400 days). Neonatal endotoxin exposure was found to significantly increase circulating levels of corticosterone on postnatal days 3 and 5 at 4 h postadministration (P < 0.05). Additionally, endotoxin exposure was found to markedly alter anxiety-like behaviour in adulthood and senescence (P < 0.05). Specifically, adult and senescent endotoxin treated animals displayed significantly more anxiety-like behaviour than vehicle treated controls. Interestingly no significant differences in anxiety-like behaviour were observed between treatment groups during adolescence. These findings highlight the importance of the early life microbial environment in the development of emotional behaviour and suggests that neonatal infection may be an important predictor of susceptibility to anxiety related disorders in adult life.