This study aimed to investigate the effects of ethanol extracts of Butea superba in increasing intracavernous pressure (ICP) in vivo. The extracts were prepared from fresh and dried root cores and fresh and dried root barks. Penile erection was induced in aged rats by electrical stimulation of the cavernous nerve. Cavernous smooth muscle relaxation was also observed in vitro in the presence of the extract, cGMP or isobutyl-methylxanthine (IBMX) alone or the extract together with cGMP or IBMX. The dried root core extract from Phrae was the most effective in increasing the ICP. The dose-response relationship study revealed a bell-shape curve with the maximum effective dose at 1 mg/kg. The ICP of the control and 1 mg/kg extract-treated animals were 45.3 +/- 2.5 and 100.9 +/- 14.0 mmHg, respectively. The extract, cGMP and IBMX alone induced dose dependent muscle relaxation. B. superba significantly enhanced the effects of cGMP and IBMX. The results suggest that ethanol extracts of B. superba are effective in enhancing penile erection. The dried root core extract from Phrae is the most effective part with a maximal dose of 1 mg/kg. The results also suggest that B. superba may act through cAMP/cGMP pathways.