YM348, (S)-2-(7-ethyl-1H-furo[2,3-g]indazol-1-yl)-1-methylethylamine, showed a high affinity for cloned human 5-HT(2C) receptors (K(i): 0.89 nM). The functional selectivity for 5-HT(2C) receptors in the 5-HT(2) receptor family was the highest among 5-HT(2C) receptor agonists, including m-chlorophenylpiperazine (mCPP) and Ro60-0175 ((S)-2-(6-chloro-5-fluoroindol-1-yl)-1-methylethylamine). Oral administration of YM348 induced penile erections and hypolocomotion in rats, being completely inhibited by a selective 5-HT(2C) receptor antagonist, SB242084 (6-chloro-5-methyl-1-[6-(2-methylpyridin-3-yloxy) pyridin-3-yl carbamoyl] indoline). The dose-response curve for penile erections, unlike that for hypolocomotion, was an inverted U-shape in the dose range of 0.0677-2.03 mg/kg. A selective 5-HT(2A) receptor antagonist, MDL100907 (R(+)-alpha-(2,3-dimethoxyphenyl)-1-[2-(4-fluorophenylethyl)]-4-piperidine-methanol), and a selective 5-HT(2B) receptor antagonist, RS-127445 (2-amino-4-(4-fluoronaphth-1-yl)-6-isopropylpyrimidine), had no effect on the decline in penile erection frequency at 2.03 mg/kg of YM348. YM348 did not affect blood pressure at 2.03 mg/kg. In conclusion, YM348 is a novel, potent and orally active 5-HT(2C) receptor agonist, and neither the activation of 5-HT(2A) or 5-HT(2B) receptors nor a cardiovascular effect is likely to contribute to the inverted U-shape dose-response curve for penile erections.