The effect of hypothalamic deafferentation upon levels of LH-RH in brain, cerebrospinal fluid, and plasma was studied in adult male rats. Deafferentation produced a significant increase in the LH-RH concentration of third ventricular CSF (from 140.6 to 343.9 pg/microl), while there was no change in median eminence or hypothalamic content. LH-RH in the pineal gland and the posterior pituitary was also elevated by hypothalamic isolation.