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We have used in vivo microdialysis in anaesthetised rats to investigate whether somatostatin (SRIF) can play a neuromodulatory role in the striatum. When 100 nM SRIF was retrodialysed for 15 min, it increased concentrations of dopamine (DA) by 28‐fold, γ‐aminobutyric acid (GABA) by eightfold, and glutamate (Glu) by sixfold as well as those of aspartate (Asp) and taurine (Tau). These effects were both calcium‐ and tetrodotoxin‐sensitive. Lower (10 or 50 nM) and higher (1 µM) SRIF concentrations were less effective. Rapid sampling showed that whereas Asp and Glu concentrations were raised for 3 min at the start of 15‐min SRIF infusions, those of DA were increased for 12 min. A second 15‐min application of 100 nM SRIF given 135 min after the first application failed to increase transmitter release. An NMDA receptor antagonist, 2‐amino‐5‐phosphonopentanoic acid (200 µM), blocked SRIF (100 nM)‐evoked …