A nuclear hepatic estrogen receptor was characterized in the female turtle Chrysemys pictu for the first time. The characteristics of this receptor were similar to those previously described in the cytosol, using DNA-cellulose column chromatography (Riley et al., '87). The development of a different method for measurement of turtle cytosolic estrogen receptors allowed comparison of cytosolic and nuclear estrogen receptor regulation, Cytosolic and nuclear estrogen receptors exhibited high affinity (Kd, nuclear, 2.3 x lO-'M), limited capacity (10 -170 fmoVg liver), steroid specificity for natural and synthetic estrogen, and target tissue specificity. Sucrose gradient analyses of crude cytosol revealed specific peaks of radioactivity at 4s and 9S, however a single 4s peak was present in nuclear extracts and ammonium sulfate precipitated cytosol. The levels of liver nuclear estrogen receptor changed significantly during the annual reproductive cycle, while cytosolic receptors were maintained at a constant level. Vitellogenic animals had higher levels of nuclear estrogen receptor and a greater percentage of total receptor in the nuclear fraction. In nonreproductive animals with low nuclear and high cytosolic estrogen receptor, short term (20 h) or chronic (1 week) estrogen exposure resulted in an increase in nuclear receptor to levels similar to naturally vitellogenic animals; however, depletion of cytosolic receptor was observed. Hypophysectomy of reproductively inactive animals resulted in loss of nuclear estrogen receptors in both control and estrogen-injected turtles and restoration was achieved by ovine growth hormone (oGH) replacement. Cytosolic estrogen receptor levels and depletion after estrogen treatment were unaffected by hypophysectomy, however an increase above control occurred in hypophysectomized turtles treated with oGH. These data suggest that both estrogen and a pituitary factor similar to oGH are necessary for the achievement of high levels of nuclear estrogen receptor associated with vitellogenesis.