ADHD has been sometimes associated to a defective interhemispheric cross-talk caused by hypoplasia of the corpus callosum. The inbred mouse strain I/LnJ shows total callosal agenesis with complete penetrance, and behavioral features which resemble ADHD. In conditioned learning tasks, as well as in paradigms of spontaneous behavior. I/LnJ mice, as compared to other inbred strains, show lower learning scores, impulsiveness, and significantly higher locomotor activity, albeit with considerable individual variations. In order to disentangle the influences of the genetic background from the effects of the callosal agenesis, we undertook crossing studies between I/LnJ and C57BL/6 mice, obtaining hybrids with missing corpus callosum. In comparison to normal C57BL/6 mice, acallosal hybrids exposed to a novel open-field showed a different locomotor pattern, with less short stops and more center crossing during the beginning of the session. In a metabolic mapping study, the tendency of acallosals to stay off the walls was found to be associated to lower 2-deoxyglucose uptake in the left striatum and cerebral cortex, while the number of short stops was correlated to the bilateral levels of 2-deoxyglucose uptake in the frontal and parietal cortex. The results hint at a right hemisphere dominance in impulsiveness and hyperactivity, boosted by the lack of callosal connections.