Abstract Rat brain slices containing the nucleus tractus solitarius loaded with [ 3H]5-hydroxytryptamine ([ 3H]5-HT) for superfusion were stimulated at (3 Hz, 25 mA, 1 min) resulting in fractional release ratios S 2 S 1 of 0.89 for [ 3H]5-HT in the presence of the serotonin uptake inhibitor 1.0 μM 6-nitroquipazine (6-NQ). α-Methylserotonin (α-Me-5-HT; 1.0 μM), a non-selective 5-HT 1 and 5-HT 2 receptor agonist, significantly reduced the S 2 S 1 ratio of [ 3H]5-HT without affecting the basal release ratios B 2 B 1 1.00 ± 0.04. Without 6-NQ in the perfusion medium 1.0 μM α-Me-5-HT sharply increased the basal release B 2 B 1 to 2.21 ( P < 0.01). In low Ca 2+ medium the S 2 S 1 ratio was reduced to 0.06 and α-Me-5-HT promoted a B 2 B 1 release of 2.17 ( P < 0.01). The 5-HT 3 antagonist LY-278,584 did not block α-Me-5-HT induced basal release of [ 3H]5-HT. Both pindolol and LY-53,857 blocked the autoinhibitory effects of α-Me-5-HT, but only LY-53,857 and 6-NQ blocked the basal release induced by α-Me-5-HT. These results suggest that α-Me-5-HT reduces neurotransmitter release through the serotonin autoreceptor and displaces serotonin through a non-exocytotic release mechanism.