The lactic acid/pyruvic acid quotient of rat anterior pituitary varies betweeen 30 and 80. The dominance of lactic acid is in accordance with the earlier observation that pituitary lactate dehydrogenase isoenzymes in rat are made up, in majority, of M subunits. The pituitary lactic acid concentration in female and male animals shows no appreciable difference and does not change 2 and 4 weeks after castration. In males the pituitary pyruvic acid concentration was almost double of that in females. Pyruvic acid content increases after ovariectomy and decreases after orchidectomy. The sexual differences having been observed in lactic acid/pyruvic acid quotient are in correlation with the discrepancy of pyruvic acid concentration.