Previous research into the subsequent effects of prenatal methadone exposure has primarily focused on neurological changes and short term physical development. While there have been some studies of behavioural development, only short term effects have been investigated. The present research therefore aimed to assess longer term behavioural development of offspring exposed to methadone gestationally, lactationally or both. Methadone was provided in the drinking water of drug-treated rat dams during gestation (2.39mg/kg/day) and lactation (2.86mg/kg/day). The four conditions were: non-exposure/control (N = 24), gestational-exposure (N = 20), lactational-exposure (N = 24), and combined-exposure (N = 21). As well as several measures of pregnancy characteristics, offspring postnatal physical development was assessed at 30, 60 and 120 days after birth. Behavioural assessments were also made at these ages by means of an open-field, Y maze and emergence apparatus. There were no significant differences in physical development. Maternal methadone exposure during gestation reduced the number of rat dams that became (or remained) pregnant. In the offspring, there was increased activity in lactationally-exposed rats through into adulthood. Anxiety was increased in the combined-exposure condition, primarily in adolescent males. The significant longer term effects of earlier methadone on the rats’ behavioural development supported the need for more research into this hitherto relatively neglected area. More information about effects of methadone exposure on anxiety and activity, as well as on social functioning and motor coordination could be useful for understanding potential risk factors in the ever growing methadone-exposed population, and thus suggesting best practice for methadone maintenance programmes.