It has been suggested that somatostatin is involved in nociceptive transmission in the dorsal horn and that it is contained in small primary afferent neurons. In the present experiments, to elucidate neural systems inhibiting the release of somatostatin from the primary afferent terminals, we examined the effects of serotonin, noradrenaline and gamma-aminobutyric acid on the capsaicin-evoked, dorsal-rhizotomy-sensitive and tetrodotoxin-insensitive release of immunoreactive somatostatin, 98.7% of which was somatostatin itself, from the dorsal-half slices of lumbar and cervical enlargements of rat spinal cord. Serotonin (30-100 microM) suppressed the evoked release in a concentration-dependent manner, and the suppression was antagonized by methysergide (100 microM). The evoked release of immunoreactive somatostatin was not inhibited by noradrenaline (100 microM) or gamma-aminobutyric acid (100 microM). The present results suggest that the serotonergic systems exert an inhibitory effect on the release of somatostatin from the central terminals of primary sensory neurons.