Sex differences are observed in various physiological, behavioral, and psychic functions, including reproductive behavior, aggres sion, emotions, and cognition. Such differences are expressed even in early childhood with preferences to definite activities. It has been generally accepted that differences between genders are formed under the influence of biological as well as environmental factors. The existence of sex differences in functions of the central nervous system has suggested that there are also morphological sex differences. In recent years several reports on sexual dimorphism in the brain of vertebrates have been published. However, the mecha nisms of sexual differentiation of the central nervous system remain unclear in most cases. It is often difficult to correlate morphological sex differences to differences in definite function or behavior. We set out to explore the sexual dimorphism of the limbic system and especially the bed nucleus of the stria terminalis and the amygdala, which are considered generally to be occupied with the control of reproductive behavior and autonomic and compli cated psychic functions. Several reports on sexual dimorphism of these structures have been published. Some of them have been directed to the total neuronal number and the volume of the nuclei, while others have concentrated on definite subpopula tions of neurons. In many cases the mechanisms of sexual differ entiation were tested, but sometimes they could not be estab lished."