It is currently believed that sexual differentiation of the brain is mediated entirely by the epigenetic action of gonadal steroids during a critical period of development. Ingrid Reisert and Christoph Pilgrim review sexual dimorphisms of monoaminergic systems, which also appear to be generated by sex steroids. However, there are a number of observations that are not explainable by the 'androgen theory of sexual differentiation'. Results obtained from cultures of embryonic rat brain tissue appear to indicate that dopaminergic neurons may develop morphological and functional sex differences in the absence of sex steroids. Hormone-independent and -dependent developmental processes may affect diencephalic and mesencephalic dopaminergic neurons in a regionally diverse fashion. Factors other than sex steroids need to be examined. It is possible that some sexual dimorphisms in the nervous system may develop under primary genetic control.