We examine the contributions of individual genes to the genetic determination of complex behavioral traits. Because brain serotonin pathways regulate many mammalian physiological and behavioral phenomena, our primary focus is on gene products that contribute to this signaling system. To shed light on the mechanisms through which serotonin regulates neurobehavioral processes such as anxiety/fear responses, ingestive behavior and reward, we use molecular genetic procedures to generate lines of mice with null mutations (constitutive, region-specific and inducible) of serotonin receptor genes. Detailed behavioral analyses of the impact of these mutations are pursued, revealing abnormalities that provide a focus for multidisciplinary studies of the neural mechanisms underlying the regulation of complex behaviors. In view of the widespread use of serotonergic drugs such as Prozac for the treatment of conditions such as anxiety and depression, we have an interest in examining the serotonergic...