The effects of protamine (6 mg kg-1) injected after heparin (5 mg kg-1) have been studied in five groups of five New Zealand white rabbits. Group I was treated with the sequence heparin-protamine and group II with protamine alone. The animals of groups III and IV received respectively intravenous indomethacin (3 mg kg-1) and BN 52021 (3 mg kg-1), a paf receptor antagonist before the sequence heparin-protamine. Group V was pre-treated with indomethacin and BN 52021 before heparin reversal with protamine. In group I, immediate thrombocytopenia (44.1 +/- 4.6% of baseline level, P less than 0.05) and leucopenia (55.5 +/- 2.3% of baseline level, P less than 0.05) were observed 30 s after protamine reversal of heparin, paralleled with an increase in blood paf levels (27.6 +/- 27.6 vs. 148.2 +/- 48.9 pg ml-1, P less than 0.05). In group II, protamine alone induced no change in platelet count nor in blood paf levels (55 +/- 10 vs. 52.5 +/- 20 pg ml-1, P greater than 0.05). Pre-treatment with indomethacin alone (group III) did not protect the animals against the haematological changes induced by the heparin-protamine complexes. Pre-treatment with the paf receptor antagonist, alone or in association with indomethacin, delayed the occurrence of thrombocytopenia 3 min after protamine administration but the leucopenia was the same as in group I. This study demonstrated that paf is implicated in the immediate thrombocytopenia occurring after protamine reversal of heparin in rabbit.