We evaluated the effect of different manipulations of central somatostatin (SS) neurons on dexamethasone (Dex) induced suppression of the hypophyseal-pituitary axis. The Dex suppression test was performed in male rats by evaluating the reduction of plasma levels of corticosterone occurring 4 h after administration of Dex. Administration of cysteamine, a depletor of SS stores, or passive immunization by intracerebroventricular administration of a saturating dose of anti-SS γ-globulins completely suppressed the ability of Dex to reduce plasma corticosterone levels. Additionally, anti-SS γ-globulins, infused under stereotaxic control into two discrete areas of the hippocampus (CA3 and DG), completely blocked Dex-induced inhibition of corticosterone secretion. These data indicate that hippocampal SS neurons play a role in the negative feedback regulation of glucocorticoid secretion.