The effects of some phosphodiesterase (PDE) inhibitors (dipyridamole, theophylline, papaverine and SH-869) on prostacyclin (PGI₂) production have been studied in vitro and in vivo. PGI₂ was bioassayed by Vane's superfusion technique. In rabbit aortic rings, only dipyridamole in concentrations from 1 to 12 µM was able to stimulate PGI₂ biosynthesis in a dose-dependent manner. This effect was also detected with so-called exhausted rabbit aortic rings. The other PDE inhibitors used, both in µM and mM concentration, did not affect PGI₂ biosynthesis. Dipyridamole was found to increase PGI₂ production in healthy volunteers, when given both by infusion (8 µg/kg/min × 2 h) and by oral administration (375 mg/day for seven days). Circulating PGI₂ and PGI₂ production induced by a 3-min period of ischaemia were increased by an average of 137% (p<0.001) and="" 30.8%="">p<0.001) respectively.="" saline="" and="" theophylline="" (as="" aminophylline)="" infusions="" used="" as="" controls="" did="" not="" affect="">₂ production.