Gonadal steroids regulate reproductive behavior in many vertebrate species, including humans. Much is known of how steroids mediate changes in anatomical attributes of brain regions that control reproductive behavior. The complexity of these brain regions, however, has made it difficult to identify physiological mechanisms by which steroids activate and modify behavior. Investigating the actions of steroids on the biophysical properties of steroid-sensitive neurons is important to understand how steroid actions on neurons modify behavior. The neural circuit controlling the sexually dimorphic electric organ discharge of weakly electric fish is an ideal system in which to study steroid actions on neurons. This neural circuit is steroid-sensitive and relatively simple. It is therefore possible to analyze the biophysical properties of the neurons that comprise this circuit, and to relate steroid actions on these properties to steroid-induced behavioral plasticity. The goal of this proposal is to investigate biophysical mechanisms that underlie steroid-regulated sexual dimorphism in the frequency of electric organ discharge. Specifically, we will determine whether androgens masculinize the electric organ discharge by changing the density or kinetics of ionic conductances in the neurons that rhythmically fire to produce the discharge. By addressing this question, we will better understand how steroid actions on neurons modify behavior and how ionic conductances contribute to neuronal oscillations that generate rhythmic behaviors.