We studied the effect of amurinone, a phosphodiesterase inhibitor, on human pulmonary arterial smooth muscle in vitro. Amurinone caused dose-dependent relaxation of pulmonary arterial strips precontracted with 60 mM KCl. Preincubation with either meclofenamate (31 microM), a cyclooxygenase inhibitor, or L-NG-nitroarginine (100 microM), an endothelium-derived relaxing factor (EDRF) production inhibitor, failed to inhibit amurinone-induced pulmonary vasodilation. The cyclic AMP (cAMP) levels in the supernatant of the lung vessel homogenates significantly increased after incubation with amurinone. These results indicate that amurinone causes relaxation of human pulmonary artery in vitro, and suggest a role for cAMP in the mechanisms of amurinone-induced pulmonary vasodilation.