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The activity of 3beta-OH-steroid dehydrogenase in the adrenal cortex of 21-day fetuses in control and 3 days after decapitation in utero was estimated quantitatively by microspectrophotometry. The enzyme activity calculated as per conditioned cell in the fascicular zone exceeded that in the forming glomerular zone. After hypophysectomy the activity of the 3beta-OH-steroid dehydrogenase decreased in both the fascicular and glomerular zones.

The concentration of corticosterone in the suprarenals after the stress effect (injection of formol) was studied in 20.5 days old foetuses of the adrenalectomized rats, using the method of competitive binding. The concentration of corticosterone in the suprarenals of normal foetuses increased reliably within 30 min after the injection of formol; this reaction was absent in the encephalectomized foetuses. The results obtained suggest the participation of embryonic hypothalamus in the organization of stress response during prenatal development of rats.

Changes in the functional state of the hypophysial-adrenal system after the removal of the hypothalamus (encephalectomy in utero) were studied in rat fetuses. The hormonal activity of the adrenal glands was evaluated by the direct fluorimetric determination of corticosterone in this organ. Removal of the hypothalamus in 18.5--19.5-day fetuses brought about a significant decrease of corticosterone in the adrenal glands. Injection of the hypothalamus homogenate to fetuses immediately after the encephalectomy prevented this decrease. The results obtained supported the existence of functional relationship between the hypothalamus and the hypophysial-adrenal system in rat fetuses.