The estrogen receptor system in a transplantable rat pituitary tumor (MtTW5) has been studied. In the tumor cell cytosol, estradiol binds to a protein which sediments primarily at SS in low salt sucrose gradients. The steroid enters the nucleus in vivo and can be extracted in bound form. The dextran-coated charcoal technique reveals very high affinity binding of estradiol to the cytosol receptor (Kd - 3 × 10-10M). Transformation of the salt-dissociated cytosol receptor from 4S to SS by warming is easily demonstrated. By the criteria examined, the estrogen receptor system in pituitary tumor MtTW5 has properties similar to that found in normal pituitary, uterus and mammary tumor. (Endocrinology93: 810, 1973)