The discovery of mammalian nitric oxide (NO) synthesis from L-arginine has led to a profound increase in our understanding of basic physiological and pathophysiological processes. The NO-dependent regulation of steroidogenic cytochromes P450, such as aromotase, and local, tissue-specific regulation of NOS isoform expression by sexual steroid hormones, such as estrogen and progesterone, promises to provide new insights into the regulation of endocrinological processes. Research on the interaction between the NO and sexual steroid systems are likely to lead to novel therapeutic concepts for use in diseases associated with perturbations of the endocrine system.