Somatostatin (SRIF) pathways in rat brain were investigated by measurement of radioimmunoassayable SRIF in median eminence (ME), preoptic area (POA), amygdala and parietal cortex (Cx) after selective brain lesions or hypothalamic deafferentation. Bilateral lesions were placed in the ventromedial nuclei (VM), anterior hypothalamic area (AHA), ventral medial preoptic area (v-MPOA), dorsal medial preoptic area (d-MPOA) or amygdala. AHA and v-MPOA lesions resulted in a 90 and 80% decrease in SRIF in the ME, respectively. VM lesions produced 50% reduction in ME and POA SRIF concentration, but only the decrease in POA was statistically significant. No effect was obtained on the SRIF content in extrahypothalamic brain regions after VM, AHA, or v-MPOA lesions. d-MPOA and amygdala lesions did not affect the SRIF content in any region assayed. MBH deafferentation resulted in a 90% decrease in SRIF content in the island and an 80% decrease in the POA. These results indicate that SRIF present in the ME arises mainly from the AHA and v-MPOA. SRIF contained in amygdala and Cx does notarise from hypothalamic neurons. The latter observation supports the view that brain SRIF may act as a modulator of neuronal activity in the central nervous system, independent of its role in GH regulation.