SYNOPSIS. Hormone retention by various target tissues has been demonstrated for a number of sex steroids. Retention of the steroid by the target tissue appears to be specific and saturation occurs at physiological levels of hormone. Thus, target tissues are said to possess specific receptors for the hormone. Only the estrogens have been extensively studied, and both a cytoplasmic and nuclear receptor have been described for various estrogen target tissues (uterus, pituitary, hypothalamus). The receptor concept has been utilized by a number of laboratoriesas a mechanism which might be helpful in understanding the relationship between estrogen effects, i.e., changes in physiology or behavior resulting from estrogen treatments. The basis for the estrogen receptor is reviewed along with the data which attempt to relate hormone retention to a physiological response. There is considerable evidence that hormone retention is a primary step in the tissue hormone interaction as in its absence the effects attributable to the presence of the hormone are not observed. Progesterone retention has also been demonstrated forsome target tissues, and an estrogen (priming) component is apparent for uterus and vagina. Todate there is no concrete evidence lor specific progesterone retention at the neural level.