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The 2-deoxy-D-glucose method was used to study asymmetries in cerebral metabolic activity in neonatal rats. Left-right asymmetries in 2-deoxy-D-glucose uptake were observed in hippocampus, diencephalon, cortex, and medulla-pons: 2-deoxy-D-glucose incorporation was greater in right hippocampus, right diencephalon, left cortex, and left medulla-pons. These asymmetries occurred only in females. We also observed neonatal asymmetries in tail position that, in both sexes, were predictive of adult turning preferences; females had right-sided biases in both neonatal and adult characteristics. Collectively these data indicate that cerebral lateralization is sexually dimorphic and is present at birth.Cerebral lateral asymmetry was once believed to be a unique attribute of humans (1) but can no longer be considered as such. An ever-increasing body of knowledge clearly indicates that the brains of normal animals are lateralized (2) and that this lateralization is oftentimes manifest behaviorally (e.g., refs. 3-5). Thus, it has been shown in this laboratory that spontaneous side preferences and nocturnal (6) and amphetamine-induced (7) rotation are related to an intrinsic asymmetry in dopamine content (8), metabolism (9), and dopamine-stimulated adenylate cyclase activity (9) in the corpus striatum of rats. In addition, it has been demonstrated that rats have asymmetries in selfstimulation thresholds that are also related to rotation (10).Moreover, we have recently shown that there exist asymmetries in 2-deoxy-D-glucose (dGlc) incorporation in several regions of the adult (11) and neonatal (12) rat brain.It is known that the human brain is characterized by an asymmetry in temporal lobe gross anatomy and cytoarchitectonics and that this asymmetry can be observed prenatally (13). Other studies have revealed asymmetries in head posture in human neonates during the first 24 hr of life (14). With these findings in mind, coupled with those that suggest that asymmetry in the rat is in some way reflective of that in the human, we thought it of considerable interest to determine whether the rat brain is lateralized at birth and if this lateralization is in any wav related to behavioral phenomena. To study these questions we observed the behavior of neonatal rats during their first week of life and measured dGlc incorporation in various bilaterally dissected brain regions. The present study details behavioral and biochemical findings that, collectively, suggest that cerebral asymmetry is present at birth, is related to an asymmetry in tail posture, and is sexually dimorphic. MATERIALS AND METHODSSubjects. Male 40.0 Ci/mmol; 1 Ci = 3.7 x 10'°becquerels) was dissolved in physiological saline and administered intraperitoneally to rats at a dose of 2 uCi/g of body weight in 50 pL. Three males and three females were used on each day from birth (day 0) through day 7. The rats were removed from their mothers, weighed, quickly injected with isotope, and maintained at 370C by using a rheostat-controlled heat lamp. Thirty minutes af...