We have demonstrated recently the formation of a biologically active metabolite of prostaglandin (PG) E1, 13,14-dihydro-PGE1, during intravenous infusions of PGE1 in patients with peripheral arterial occlusive disease. We have now investigated the levels of the immediate precursor of 13,14-dihydro-PGE1, the biologically inactive 15-keto-13,14-dihydro-PGE1, during intravenous administration of 20 micrograms, 40 micrograms or 80 micrograms PGE1 over a period of 60 min to human volunteers. It was found that levels of 15-keto-13,14-dihydro-PGE1, but not those of PGE1 itself, increased in a dose-dependent manner. Thus, increased formation of 13,14-dihydro-PGE1 from 15-keto-13,14-dihydro-PGE1 with increasing doses of PGE1 can be expected to occur. It remains to be investigated, to which extent formation of small amounts of 13,14-dihydro-PGE1 during intravenous infusion of PGE1 could contribute to the therapeutic effects of PGE1 in patients with peripheral arterial occlusive disease.