Several members of the family of receptors coupled to G proteins have been cloned in recent years. From the primary sequence information furnished by these clones, a characteristic seven-membrane spanning topography has emerged as the prototype for this class of receptors, in analogy with the opsin visual pigments. Cloned genes for the various receptors provide important tools for probing the regulation of their expression. For example, the underlying genetic basis for the expression of adrenergic receptor subtypes can now be explored. The physiological regulation of receptors by heterologous hormones, such as steroids and thyroid hormones, has long been suspected to involve changes in the expression of the relevant adrenergic receptor genes. At least for the glucocorticoids, beta 2AR expression is controlled at the level of transcription. The hormonal regulation of other adrenergic receptor subtypes is currently being explored.