The use of newborn or immature animals in biomedical research poses certain challenges, in that the ecological niche of immature animals differs from that of adults, and the stimuli provided to immature animals can have profound physiological and behavioural effects through adulthood, even at the cerebral level. In particular, the newborn's behaviour and physiology are regulated by olfactory, thermal, and tactile stimulation supplied by the mother in the nest environment. Thus any disturbance to the mother-offspring relationship in the first two weeks of life can profoundly change the physiological and psychological state of the young. For this reason, research experiments must be adapted to the specific characteristics of the immature animal, to improve not only the quality of the data obtained but also the welfare of the animals. To this end, the present work provides some ethological and methodological considerations in the maintenance, handling, and testing of newborn rats and mice in biomedical research.