The effect of altering the prenatal hormonal environment on behavioral development and neuroendocrine function was studied in genetic males and females exposed to either and androgen receptor blocker (flutamide), an long-lasting androgen (testosterone cypionate), or vehicle controls. Treatments were given at the beginning of the second (early) or third trimester (late). Only early flutamide treatment produced substantial genital modification to treated subjects. Early androgen treatment reduced neonatal responsiveness to exogenous gonadotropin-releasing hormone (GnRH) in females, while late flutamide treatment increased responsiveness and basal LH secretion. Behavioral and neuroendocrine development of these unique monkeys will continue through puberty and into adulthood. This work provides fundamental information about the role prenatal androgenic hormones play in the sexual differentiation of behavior and endocrine function and is relevant to understanding in humans how small modifications in the early hormonal environment affect later development.