There have been many reports of the action of morphine in reducing the release and the turnover of acetylcholine and biogenic amines both in vivo and in vitro [l--5]. Similarly enkephalins have been shown to inhibit the release of acetylcholine and biogenic amines from various in vivo and in vitro preparations 16-101. These effects are mostly prevented by naloxone. Release of amino acid neurotransmitters and acetylcholine from the motor cortex of awake, behaviourally normal rats in vivo was suppressed by morphine given intraperitoneally, the effect being prevented by naloxone. This release was evoked either by low concentrations (1 PM) of the depolarizing scorpion venom toxin, tityustoxin delivered onto the cortical surface [Ill, or by sensory stimulation of the motor cortex via the brachial plexus 1121. Synaptosomes from rat cerebral cortex [ 141 were incubated at 37°C for 15 min in Ca2+-free Krebs-Tris medium of the following composition (mM): NaCI, 138; KCl, 5; MgS04,1.0; Tris, 20 (pH 7.4); glucose, 10; before adding morphine sulphate at the levels indicated. After a further 15 min incubation, veratrine (a depolarizing alkaloid) was added as designated in table 1. Another 10 min incubation was allowed before 45Ca2+ (Amersham International) was added to give 1.2 mM final cont. and spec. act. 0.8 mCi/mmol. After 10 min the synaptosomes suspensions were added to ice-cold EGTA (final cont. 15 mM) and centrifuged immediately. The supernatants were stored for amino acid analysis whilst the pellets were taken for analysis of 45Ca2+ content (151. Amino acids were measured by autoanalysis as in [ 161.