INTERACTION OF HORMONES with target organ cells requires that the hormones first bind to the cells; this initiates the chain of events that represent the effects of the hormone. In recent years much information has been gathered about the nature of this initial binding interaction? The hormones first attach to specific discrete molecules (proteins or glycoproteins) that have a high affinity for the hormone. These molecules are hormone receptors. Receptors for the polyeptide hormones insulin, growth hormone, gonadotropins, thyrotropin, ACTH, glucagon, and parathormone are located in the cell (plasma) membrane. The term" plasma membrane" is used to distinguish it from others, such as the mitochondrial or nuclear membranes. Steroid hormones (cortisol, aldosterone, estradiol, testosterone, dihydrotestosterone) and 1, 25 (OH),-vitamin D receptors are located in the cytoplasm. The steroid hormone-receptor complex migrates to the nucleus after it is formed, and exerts specific influences on the nuclear control of protein synthesis. The thyroid hormones do not require a cytoplasmic intermediary and appear to associate directly with nuclear chromatin, t Receptors for epinephrine and norepinephrine are located in the plasma membrane. Receptors for substances other than hormones that serve to convey stimuli across cell membranes also