An injection of estradiol benzoate (EB, 5 microgram/rat) to ovariectomized rats suppressed serum LH levels 2, 4, 8 and 24 h later; serum LHRH levels were unaltered but the LHRH content in the medial basal hypothalamus (MBH) was significantly elevated at 24 h. EB treatment also modified the sensitivity of the pituitary to LHRH administration: a rapid decrease at 4 h followed by an augmentation in LH release at 8 and 24 h. 3H-Leucine incorporation into proteins of the pituitary and neural tissues was unaffected for 4 h following EB treatment at the time when serum LH and the responsiveness of the pituitary to LHRH were clearly depressed. However, a marked increase in 3H-leucine incorporation into the proteins of the pituitary alone occurred at 8, 16 and 24 h after EB treatment, coincident with the enhanced pituitary sensitivity to LHRH. These studies showed that estrogen promoted storage of LHRH in the MBH and alteration in protein metabolism may be an important initial step in the estrogen-induced augmentation of the pituitary responsiveness to LHRH.