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The binding of [3H]5 alpha-androstane-3 beta, 17 beta-diol (3 beta-Adiol) in the pituitary cytosol of prepubertal male rats was studied. The following criteria indicated that 3 beta-Adiol was bound to specific proteins and, in all probability, to the estrogen receptor. 1) The binding was of relatively high affinity and low capacity. 2) The complex was destroyed by proteolytic enzymes and by heating. 3) It was precipitable with protamine sulfate and 35% ammonium sulfate. 4) In sucrose linear gradients (5-20%), 3 beta-Adiol was bound in the 7S region. The 3-4S 3 beta-Adiol complex, previously found, was due to the dissociation of 3 beta-Adiol from the 7S complex and further association with nonspecific proteins. 5) Bound hormone was only displaced by 3 beta-Adiol itself and estrogens. 6) Translocation of the estrogen receptor from cytosol to nuclei resulted in a parallel decrease in the number of cytosol 3 beta-Adiol-binding sites. It was also shown that 3 beta-Adiol was not bound to contaminating serum proteins, particularly to alpha-fetoprotein. We interpret these results to indicate that in the male rat pituitary, 3 beta-Adiol is bound to the estrogen receptor.

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