The Indiana lines of selected rats, the HAD and LAD replicates and the P and NP lines, were bred for high and low alcohol preference. The P and HAD lines have met criteria for an animal model of alcoholism in that they voluntarily consume sufficient ethanol to achieve significant blood alcohol concentrations, and their alcohol-seeking behavior is reinforced by the pharmacological effects of ethanol rather than its taste, caloric content, or other properties. These lines have been characterized extensively for associated behavioral and physiological phenotypes. The P and HAD rats show an enhanced responsiveness to the stimulatory effects of ethanol and reduced sensitivity to the aversive sedative effects of ethanol. Consistent findings with the selected lines include differences in the mesolimbic dopamine reward system, as well as differences in serotonin, GABA, endogenous opioid, and neuropeptide Y systems. Genetic mapping studies have identified quantitative trait loci influencing alcohol preference on chromosomes 3, 4, and 8 in the inbred P/NP rats and on chromosomes 5, 10, 12, and 16 in the noninbred HAD1/LAD1 rats. The elucidation of the genotypes and phenotypes that result in excessive alcohol intake may lead to a better understanding of alcohol abuse and alcoholism and could guide strategies for potential treatment and prevention.