The pituitary endocrine function in 4 anencephalic infants was studied by measuring the concentrations of growth hormone, prolactin, thyroid-stimulating hormone (TSH), follicle-stimulating hormone (FSH) and luteinizing hormone (LH) in umbilical venous serum. There was no significant difference in hormone levels between the anencephalic and normal infants. The response of FSH and LH was negative after administration of 10 μg of luteinizing hormone-releasing hormone. The TSH levels after administration of 50 μg of TRH elevated about 4- to 6-fold over pre-administration levels. These results indicate that the hypothalamo-pituitary axis of the human fetus initiates its function from mid-pregnancy; TSH may have an autoregulating system of the pituitary itself.