In order for animals to experience pain and to suffer from it, they have to be capable of conscious perception. Recent evidence suggests that the fetus is maintained in a sleeplike unconscious state and that conscious perception therefore only occurs after birth. The timing of the onset of conscious perception depends on the maturation of underlying neurological processes and is anticipated to be species dependent. Painspecific electroencephalographic (EEG) responses of lightly anaesthetised young of three species born at different levels of neurological development were investigated. The results of the present thesis are in agreement with published data on general neurological, EEG and behavioural development. This information, in addition to the present results, has been used to estimate the approximate time of the onset of conscious perception in tammar wallaby joeys, rat pups and newborn lambs. In wallaby joeys (extremely immature at birth), the EEG remained isoelectric until about 100-120 days of in-pouch age and became continuous by about 150-160 days, with electroencephalographic and behavioural signs of conscious perception apparent by about 160-180 days. In rat pups (immature at birth), the absence of a differentiated EEG suggests that the ability for conscious perception in pups younger than 10-12 days is doubtful. The marginal EEG responses to noxious stimulation in 12-14 day-old pups and the pronounced EEG responses in pups 18-20 days suggest that rats may be capable of conscious perception from 12-14 days onwards. In lambs (mature at birth), full conscious perception is probably not apparent before 5 minutes after birth and may take up to several hours or days to become fully established. Its modulation by the residual neuroinhibitor allopregnanolone, if that occurs, would be highest over the first 12 hours after birth. Overall, the onset of conscious perception does not seem to follow an “on-off phenomenon”, but seems to develop gradually, even in species born neurologically mature. Although conscious perception, and hence pain experience, may be qualitatively different in younger animals, on the basis of the precautionary principle, when significantly invasive procedures are planned, pain relief should be provided from those postnatal ages when pain may first be perceived – i.e. from about 120 days in the tammar wallaby joey, about 10 days in the rat pup and from soon after birth in the lamb.