Using drugs such as alcohol and morphine among others can be addictive in some individuals, and progress into a substance abuse disorder. The mesolimbic dopaminergic system (MD-system) is involved in the reward process during the development of drug addiction. The MD-system is critical for survival and affects different behaviors in both man and animal. Neurochemical pathways drive for instance physical activity, food intake, love and reproduction and are part of the natural reward process involved partly in the release of dopamine (DA) into frontal lobes. Within the MD-system opioid receptors throughout the brain are affected by drug intake, and activation of these receptors modulate DA-release in brain regions involved in reward-behavior. The aim of this study was to evaluate gene expression of MOR and DOR within the endogenous opioid system (EO-system) in relation to voluntary physical activity, a natural reinforcer. Further on investigations of the drug alcohol was compared to the natural reinforcer sucrose using voluntary consumption. For both experiments qRT-PCR was used to measure mRNA levels of MOR and DOR from brain areas of interest. We found a small significant up regulation in NAc, PFC and VTA but for DOR in VTA a down regulation in gene expression of physical exercising mice. Additionally these two different genes OPRM1- and the OPRD1- gene are down regulated in VTA and NAc due to alcohol- and sugar-intake. This implicate that the natural reward system and their ORs point in the direction of earlier findings; the opioid receptors have a key role in regulate alcohol intake and the natural rewarding stimuli as food intake.