Administration of testosterone (T), 17 beta-estradiol (E2), or 5 alpha- dihydrotestosterone (DHT) to female zebra finch chicks (Poephila guttata) at hatching exerts effects on brain sexual differentiation. Within a telencephalic station (the nucleus robustus archistriatalis, RA) of the neural pathway which participates in the efferent control of song, masculinization of several indices of neuronal size is induced by exposure to T or E2. Within RA, a sensitive assay of a single neuron's sexually differentiated state is the diameter of its soma. By this criteria, all of the neurons within RA can be masculinized with a sufficient dose of T. As the dose of T is progressively decreased, the proportion of RA neurons which undergo the transition from female to male falls, while the magnitude of the change in soma size remains basically unaltered. Administration of T or DHT masculinizes the number of neurons in RA.