An understanding of the basic mechanisms of hormone action is becoming an important part of a clinician's training. With the advent of radioreceptor assays and their comparison with radioimmunoassays, we are becoming increasingly aware that the normal physiological function of a hormone is not necessarily dependent on "normal" levels of hormone being present in the plasma. Even if plasma levels are normal, if a particular target cell lacks the receptor for the particular hormone the target tissue will not respond to the hormone. It has been shown, for example, by many workers that steroid hormones act on their cells via a receptor located in the cytoplasm of the target cells (1-4). Since the presence or absence of such receptors are becoming increasingly important in terms of unexplained infertility, endometrial carcinoma and breast cancer, it is necessary that the practicing clinician be familiar with the concept of receptors and have some understanding of their mode of action. In this brief presentation, I will explain certain terminology and summarize the state of the art so that you can critically read the literature concerning new developments in the area of hormone action which is to become increasingly important in the next few years. I will discuss some of the aspects of the mechanism of peptide hormones such as LH and FSH but will devote most of my attention discussing the details of steroid hormone action since our knowledge in this area is much more complete. I will also explain the terms frequently discussed in the literature concerned with hormone-receptor interactions.