SYNOPSISSimultaneous measurement of luteinizing hormone (LH) in the pituitary and in circulating blood is a necessary step for the determination of its storage and secretion patterns during the estrous cycle, by which the critical time of preovulatory surge of LH would be determined. Since a sensitive and specific assay method for LH, the ovarian ascorbic acid depletion test, was established by Parlow (1958Parlow ( ,1961, the changes in pituitary LH content during the estrous cycle were determined by Mills and Schwartz(1961) and by Schwartz and Bartosik(1962) and the fluctuations in plasma LH concentrations during the estrous cycle were measured by Ramirez and McCann(1964). These results indicate that either decrease in pituitary LH or increase in plasma LH occurs in the afternoon of proestrus. Recently, simultaneous measurement of plasma LH and pituitary LH of rats during estrous cycle was made by Anderson and McShan(1966) who demonstrated that the marked increase in plasma LH concentration was associated with the notable decrease in pituitary LH content in the afternoon of proestrus stage. All the abovementioned experiments, however, have not yet precisely determined the critical time of ovulatory release of LH, although it was indirectly assumed to be between 2 and 4 p.m. of proestrus by the early experiment of Everett et al.(1949).In this study, an attempt was made to determine the concentration of LH in the

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