Neurogenetic and molecular neurobiological investigations of sex-specific behavior in Drosophila are proposed. These studies will focus on two of the most salient, quantifiable, and behaviorally important features of the fixed action patterns that define these phenomena: the male's species-specific courtship song and separate components of mating itself. Among the experiments proposed are those involving manipulation of genes that function within the sex-determination hierarchy (SDH) of Drosophila melanogaster: fruitless (fru) and doublesex (dsx). The fru gene will be manipulated to delve into the neural substrates of courtship song, which cannot be performed by fruitless mutants lacking male-specific FRU protein in their ventral nerve cord (VNC); and of mating biology, two elements of which are dramatically perturbed in certain fru mutants. For the other SDH factor (dsx), similar transgene-based experiments are proposed to study the manner by which doublesex participates in programming VNC development and (potentially) regulating its ongoing function--both in the context of a sharply defined abnormality of singing behavior in dsx mutants. The doublesex experiments will be performed against a backdrop of an all-too-common view that sexual differentiation of Drosophila's nervous system is not influenced by this gene--which is belied by the behavioral phenomenon just referred to, along with a recent finding that DSX proteins are found in distinctive patterns within the metamorphosing and mature VNC. With respect to distal genic effectors of courtship actions, it is proposed to assess the effects of a fruitless-associated neurochemical factor (serotonin, or 5HT) that is strongly surmised to coordinate a key feature of mating, along with analyzing genes encoding 5HT-synthetic enzymes that are candidates to be controlled by the FRU transcription factor; and to manipuate neural expression of a song-controlling ion-channel gene (slowpoke). These SDH and effector-related experiments will involve applications of extant mutants (including those recently induced); genetic mosaics; and both spatial plus temporal manipulations of these genes' expressions, via utilization of existing transgenes and creation of novel molecularly engineered, reproductive-related factors, including those cloned from different Drosophila species and bioassayed in D. melanogaster