This study deals with the effects that are produced by an enriched early experience on the mitochondrial fraction on the mice brain. The total content of lipids and proteins in mitochondrial fraction, the morphology of the mitochondries and the density of synaptosomes were investigated. A significant difference in the total quantity of lipids of the mitochondrial fraction and the density of synaptosomes was observed between mice subjected to an enriched environment and others subjected to an impoverished environment. These mice, which were subjected to a psychological study before proceeding to the extraction of the mitochondrial fraction, displayed distinct characteristics. The enriched group displayed a greater exploratory behaviour and a greater capacity to learn than the mice raised in the impoverished environment.