To identify potent dual 5-HT2B and 5-HT7 receptor antagonists, we synthesized a series of novel carbonyl guanidine derivatives and examined their structure–activityrelationships. Among these compounds, N-(9-hydroxy-9H-fluorene-2-carbonyl)guanidine (10) had a good in vitro profile, that is, potent affinity for human 5-HT2B and 5-HT7 receptor subtypes (Ki = 1.8 nM and Ki = 17.6 nM, respectively) and