Platelet activity is regulated through synthesis and degradation of the intracellular second messengers cAMP or cGMP. The antiplatelet effect of the phosphodiesterase (PDE) III inhibitor Piroximone (PIR) was studied in vitro in platelet rich plasma. ADP induced aggregation was inhibited by PIR with an IC50 of 67 +/- 43 microM. The inhibitory effect was time and dose dependent. The antiaggregatory effects in vivo were studied in anaesthetised rats. Reduction of platelet count following injection of 100 micrograms/kg bw collagen was measured after bolus injection of PIR and vehicle. Piroximone bolus 2 mg/kg bw resulted in a 50% inhibition of platelet aggregation in rats. Cyclic AMP levels in washed platelets rose time and dose dependently after PIR. Coincubation of PDE III inhibitor PIR and adenylate cyclase activator Iloprost (ILO) resulted in a significant synergistic enhancement of the antiaggregatory effect. The PDE III inhibitor PIR exerted an effective inhibition of platelet aggregation in vivo and in vitro. The inhibitory effects in vitro were synergistically augmented by the prostacyclin analog Iloprost. These platelet inhibitory effects might be of clinical importance.