Corticosterone-4-14C, Na131I, ovine LH-KI1I, rat prolactin-131I, or bovine hemoglobin- 131I was injected into the third ventricle of rats. In 30 min, significant quantities of corticosterone- 4-14C, Na131I, and LH-131I were recovered in the pituitary stalk blood of the recipient animals. At the same time, little radioactivity was present in arterial blood. The radioactivity recovered in stalk blood after ovine LH-131I was injected into the third ventricle was associated with a substance which reacted with an antiserum to ovine LH, indicating that LH per se passed from CSF to stalk blood. These data show that certain compounds in the CSF of the third ventricle can pass through the median eminence and enter hypophysial blood.
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