Effects of neuroactive peptides on the release of labeled 5-hydroxytryptamine (5-HT) from preloaded rat spinal cord slices were investigated. The 5-HT release was significantly stimulated by somatostatin (10-50 μM) and substance P (10-50 μM), but not by neurotensin (50 μM), 8-endorphin (30 μM) and methionine-enkephalin (met-enk) (100 μM). Somatostatin-stimulated 5-HT release was markedly inhibited by γ-aminobutyric acid (GABA) (30 μM), but not by baclofen (30 μM) and met-enk (100 μM). Substance P-stimulated 5-HT release was strongly inhibited by GABA (30 μM) and baclofen (30 μM), but not by met-enk (100 μM). High concentrations (20 mM) of potassium also stimulated 5-HT release. The high potassium-stimulated 5-HT release was not affected by GABA (30 μM) and met-enk (100 μM). These results suggested further evidence on the important role of somatostatin and substance P as modulators of serotonergic neurones.