Oviduct nuclei from laying hens, or from chicks given an optimal dose of estrogen, contain receptors with a high affinity for 17beta-estradiol (apparent Kd of about 3.7 nM) at a concentration of about 10,600 molecules per tubular gland cell. Chicks withdrawn from estrogen stimulation exhibit a 7-fold lower level of nuclear estrogen receptors. Radioimmunoassay of 17beta-estradiol in the serum indicates that birds withdrawn from estrogen stimulation have a serum level of 0.09 nM. A maximum concentration of oviduct nuclear receptors is achieved when the serum level reaches 0.7 nM 17beta-estradiol. Endogenous 17beta-estradiol in the serum of laying hens is also approximately 0.7 nM. The concentration of nuclear estrogen receptors achieved by administering different dosages of 17beta-estradiol, 17beta-estradiol-benzoate, or diethylstilbestrol is related to the rate of accumulation of ovalbumin and conalbumin mRNA. The rate of conalbumin mRNA production is directly proportional to the concentration of nuclear receptors, i.e. half-maximal induction is obtained with about 5,300 nuclear receptors per tubular gland cell. In contrast, half-maximal induction of ovalbumin mRNA occurs when nuclear receptor levels are 80% of maximum; this is achieved with a dose of estrogen about 2.5 times that required for half-maximal conalbumin mRNA induction. These differential responses may be related either to different numbers of specific binding sites regulating the production of each mRNA, or to different affinities of regulatory sites for estrogen receptors.