Abstract Stress-induced antinociception (SIA) which is well characterized in the adult rat can also be observed in young rats, and, by varying swimming times, it can be dissociated into opioid and nonopioid forms. However, swimming ability in the rat does not fully develop until the third postnatal week and this has precluded the study of swim SIA in neonates. We report here the development of a harness device to aid swimming in young rats which we have successfully employed down to the age of 2 days without distress to the animals. Further we have also shown the development of the opioid form of swim SIA in the rat using this device. Swim SIA is absent at days 2 and 5, but at postnatal day 10 a small level of SIA is evident which is reversed by naloxone (10 mg/kg). Swim SIA develops rapidly thereafter, and the adult profile is observed by day 25.