Steroid hormone synthesis is controlled by two classes of mechanisms.“Acute”regulation entails rapid increases or decreases of steroid synthesis and secretion, principally mediated by rapid changes in the activities of the steroidogenic enzymes and by the availability of the substrate, free cholesterol.“Chronic”regulation entails increases or decreases in the amounts of the steroidogenic enzymes as well as their activities. The amounts of the enzymes are regulated principally by the amounts of the specific mRNAs encoding them. These, in turn, are regulated both transcriptionally and post-transcriptionally. The mRNAs are regulated by hormonal induction, by an ontogenic program, and in a tissue-specific fashion.