We determined the most suitable conditions for measuring the somatostatin (SRIF) level by brain microdialysis and investigated its release from the hypothalamus. The relative recovery rate of SRIF was 8.4 ± 0.5%(mean ± SE) using a polycarbonate (PC) membrane with the push-pull method at a flow rate of 2 μl/min. Using tubes with an internal diameter of 0.28 mm and lengths of 5, 25, 50 and 100 cm, the relative recovery rates using a PC membrane with the push method were 8.2 ± 0.5%, 7.3 ± 0.6%, 6.2 ± 0.5% and 4.1 ± 0.6%, respectively. When using tubes with an internal diameter of 0.1 mm and lengths of 5, 25, 50 and 100 cm, the relative recovery rates were 7.3 ± 0.7%, 5.6 ± 1.0%, 3.5 ± 1.1% and 1.4 ± 0.7%, respectively. The relative recovery rate was 5.2 ± 0.5% with a polysulfone (PS-F, Fresenius) membrane, 4.5 ± 0.4% with a PS-H (Hospal) membrane, 2.6 ± 0.2% with an ethylenevinyl alcohol membrane (EVAL), 5.1 ± 0.8% with a polyvinyl alcohol (PVA) membrane and 10.4 ± 0.8% with a PS-K (Kaneka) membrane. With the push method, the extracellular SRIF level in rat pituitary was 42.8 ± 1.8 pg/ml with a PC membrane, 23.1 ± 2.9 pg/ml with an EVAL membrane at a flow rate of 2 μl/min. With the push-pull method, it was 52.7 ± 5.2 pg/ml using a PC membrane, 33.5 ± 2.8 pg/ml using a PVA membrane and 54.4 ± 3.2 pg/ml using a PS-K membrane. Intraperitoneal injection of urethane significantly increased SRIF from a basal level of 36.8 ± 10.6 pg/ml to 66.6 ± 12.5 pg/ml ( P < 0.05) after 1 h; however, pentobarbital decreased it from 44.7 ± 4.1 pg/ml to 16.0 ± 4.6 pg/ml after 1 h. These data, which give optimum microdialysis conditions for measuring SRIF release in the rat, were useful for measuring the physiological release of SRIF from the hypothalamus.