The behavioural response of rat pups, 1-20 days of age, to subcutaneous injection of formalin in a rear paw is described. Formalin-injected pups were compared to handled controls and to pups that received an injection of normal saline. Ongoing behaviour was recorded every 2 min for 60 min after injection. Injection of normal saline produced little disorganization of behaviour, although day-1, -3 and -6 pups did frequently flex the limb on the injected side early in the session. Injection of 10 microliters of 1% formalin depressed active and quiet sleep in pups 10 days old and younger. Much less disruption of sleep was observed in day-15 pups, and in day-20 pups it was necessary to increase the concentration of formalin to 2.5% to produce a consistent behavioural response. The specific responses of pups to formalin injection were flexion of the limb, shaking the limb, and licking the injected paw. Pups of all ages displayed all of these responses, but in pups younger than 10 days, only limb flexion was consistent. Shaking became a consistent response in day-10 pups and licking in day-15 and -20 pups. Non-specific behaviours (squirming, vigorous rear kicks with both hind limbs and convulsive whole body jerks) were markedly increased by formalin in younger pups with a developmental pattern: squirming and kicking in day-1 pups, kicking and jerking in day-3 to -15 pups. Non-specific behaviours decreased and specific behaviours increased with age. In addition, the overall intensity and duration of the response decreased with age. The biphasic time course of the response of adult rats to formalin injection did not appear until 15 days of age.