Behavioral lateralization has previously been reported in adult animals. This work presents a study of behavioral laterality and spontaneous alternation behavior in 156 neonatal rats (39 litters with two males and two females per litter). The initial tail and head lateral movements (axis body-tail or head higher than 30 degrees) were recorded after neonatal rats were gently placed along a straight line drawn on a glass surface. This test was repeated 10 times. A leftward population lateralization was found for tail movement while head movement was rightward. The alternation behavior was lower (10-25%) than that previously reported for adult rats (80-90%) and than that expected if movement was made at random (50%). Males were more lateralized than females for head and tail movements. No sex differences were observed for behavioral alternation. In 2-day-old rats (10 litters with one male and one female per litter), the right brain side had a higher content in dopamine (mesencephalon) and DOPAC (proencephalon) than the left brain side. Thus, we concluded that behavioral and biochemical asymmetries in animals are conditioned by phenomena present during prenatal or early neonatal (first hours after birth) life and that spontaneous alternation behavior is not present during the earlier stages of postnatal development.