Steroid hormones act on the brain to influence its organization during development and its activity in adulthood, thereby regulating behavior and physiology. Achieving these effects on the brain is often the culmination of a complex set of events within the endocrine system. During development, the sequence begins with the differentiation of steroid-secreting organs and their expression of steroidsynthetic enzymes. In adulthood, it continues with the regulation of the activities of one or more of these enzymes by pituitary trophic factors. After secretion, but before the steroid reaches targets within specific brain cells, the hormone is subject to a variety of regulatory influences. These can include steps to-inactivate the molecule by peripheral catabolism and excretion. The presence of carrier proteins in blood can limit the availability of free steroid to enter tissues. Having reached a target organ, the steroid may encounter additional enzymes that catalyze changes in its structure, rendering the molecule inactive. Alternatively, the steroid may be converted to a molecule with increased biological activity or one that functions along an alternative steroid-activating pathway. When these transformations are complete, the steroid is available to influence cellular function by interacting with intracellular protein receptors. Once bound to ligand, the steroid receptor can bind to specific DNA hormone response elements to influence the transcription of specific genes. The active steroid may also influence cell function without changing gene expression by interacting directly with cell membranes or with other cellular processes.