The involvement of the 5-HT1A receptor in the 5-HT syndrome (head weaving and hindlimb abduction) induced in DBA mice by tryptamine was investigated. Methysergide, (-)propranolol and spiperone suppressed both the head weaving and hindlimb abduction induced by tryptamine. However, ketanserin and ICS 205-930 did not affect them. Haloperidol induced small decreases in the head weaving, but had no effect on the hindlimb abduction. These results indicate that the 5-HT syndrome induced by tryptamine in mice is mediated by the 5-HT1A receptor. Therefore, 5-HT syndrome may also be associated with the 5-HT1A receptor in mice, as it is in rats.