Abstract 1. 5-HT (10 −8−5 × 10 −6 M) relaxed isolated locust foreguts. 2. The effects of 5-HT were mimicked by 5,6-DHT, 5-MeOT, tryptamine, 5-MeT, MK212 and methysergide while 5-hydroxyindole, 5-hydroxyindole acetic acid, 5-hydroxytryptophol, NN-DMT, 8-OH DPAT and RU 24969 were without effect. 3. Ketanserin (pA 2 = 5.65) was a competitive antagonist of the effects of 5-HT, MK212 and methysergide. 4. Mianserin (pA 2 = 6.3) was also a competititve antagonist of 5-HT but ICS 205-930 had no antagonistic effect on 5-HT-induced relaxation. 5. It is concluded that 5-HT relaxes the locust foregut by interacting with 5-HT 2-like receptors.