Neural mechanisms underlying the regulation of ingestive behavior and energy balance are well conserved among mammals. Many neural pathways, each reflecting the function of many genes, interact to regulate these processes. Systematic genetic perturbations are not feasible in humans--the examination of gene functions relevant to feeding regulation must be performed in other species. Many advances in this field have been made through molecular genetic studies of mice, the most genetically tractable of mammalian species. The relevance of mouse ingestive behavior to the mechanisms underlying the regulation of feeding in humans is discussed. Approaches for evaluating the contributions of genes to the regulation of energy balance and to the actions of anorectic drugs are described in the context of studies focused on a line of mice lacking the serotonin 5-HT2C receptor subtype. These animal display reduced responsiveness to serotonergic anorexic drugs and a late-onset obesity syndrome associated with features reminiscent of common forms of human obesity. Developmental studies of energy balance uncovered a novel age-dependent physiological process that may contribute generally to the predisposition of humans and other mammals to accumulate fat stores during "middle-age." These findings are presented to illustrate considerations in the use of mouse molecular genetic technologies to investigate genetic influences on ingestive behavior and energy balance.