Understanding how hormones influence the nervous system helps reveal how the brain controls social behaviors like vocalization (sound production). Investigations of the simpler nervous system of fishes demonstrate that hormones dramatically influence the activity levels of individual neurons and neuron-to-neuron interactions underlying vocal behavior. Among fishes, vocal mechanisms and behaviors have largely been studied in one group known as midshipman. These studies are guided by the recent discovery that naturally occurring daily variation in midshipman vocal behavior is determined by daily changes in the activity of vocal neurons. Other studies in midshipman show that steroid hormones such as androgens and neurochemicals such as gamma-aminobutyric acid (GABA), the major inhibitory neurotransmitter in the vertebrate nervous system, induce changes in vocal neuron activity that mimic daily changes in vocal behavior. Neurophysiological, molecular and behavioral methods will test the hypotheses that daily changes in vocal neuron activity and vocal behavior depend on the actions of androgens, GABA, and the hormone melatonin that profoundly influences daily behavioral rhythms in vertebrates. The principles identified will apply to other vertebrate groups because vocal, neural and hormonal mechanisms are evolutionarily conserved between fishes and other vertebrates, including birds and mammals. This research has and will continue to train undergraduate, graduate, and postdoctoral students from diverse cultural backgrounds and geographic regions and make the results of the research easily accessible to the general public through publications, press interviews and presentations at professional societies, universities, marine biology laboratories and local community science centers.