Mammary gland slices (incubated in vitro) from hypophysectomized rats used less glucose and produced more lactic acid than similar slices from normal animals. The accumulation of lactic acid was observed in the presence of glucose or glucose plus acetate as added substrates, but not in acetate alone or in the absence of added substrates. Hypophysectomy led to a substantial decrease in the respiratory quotient of the mammary gland slices. Neither ovariectomy, adrenalectomy, thyroidectomy, parathyroidectomy nor sham hypophysectomy caused the appearance of the above metabolic changes in mammary gland slices.