The aim of the present study was to investigate antioxidativity of the triphenylethylene antiestrogen toremifene. Toremifene and its structural analogues were studied for their ability to inhibit chain reactions of lipid peroxidation and to act as scavengers of free radicals in vitro, and the effects of toremifene were compared to those of the estrogens, tamoxifen and known antioxidants. Moreover, the in vivo antioxidativity of toremifene was tested in a long-term experiment with rats. The ability of toremifene to prevent lipid peroxidation was assayed in two different test systems. In the first assay (initiated with ascorbate/ADP-FeCl3, detection by the formation of TBA-reactive material) toremifene was found to act as an efficient membrane antioxidant with an IC50-value (18 microM) comparable to that of tamoxifen (26 microM) and alpha-tocopherol (43 microM). Toremifene derivatives 4-hydroxytoremifene (IC50 = 8 microM) and Fc 1159 (IC50 = 31 microM), as well as diethylstilbestrol (IC50 = 17 microM) were also active while estradiol showed only weak antioxidativity (IC50 = 300 microM) in this test system. In the other assay (peroxidation initiated with t-butylhydroperoxide, detection by luminol-enhanced chemiluminescence) toremifene prevented lipid peroxidation only at high concentrations (IC50 = 450 microM) but the metabolite 4-hydroxytoremifene (IC50 = 0.18 microM), estradiol (IC50 = 4.6 microM) and diethylstilbestrol (IC50 = 1.7 microM) showed potent antioxidant activity. The potency of 4-hydroxytoremifene even exceded that of alpha-tocopherol (IC50 = 2.0 microM) and butylated hydroxyanisole (IC50 = 1.1 microM). Toremifene was found to have some superoxide anion but no peroxyl radical scavenging activity. Interestingly, diethylstilbestrol turned out to be a potent scavenger of peroxyl radicals. Treatment of female Sprague-Dawley rats with toremifene (12 or 48 mg/kg) was found to decrease serum levels of lipid peroxides. This was seen at various time points (2 days, 5 weeks, 6 and 12 months) during long-term administration of toremifene to rats, and results obtained with two different methods (diene conjugation, TBA-reactive material) gave similar results. The present study thus showed that (i) like steroidal estrogens and tamoxifen toremifene is a potent membrane antioxidant in vitro, (ii) the antioxidant action of toremifene is not due to scavenging of free radicals and, importantly, (iii) toremifene acts antioxidatively also in living organisms in vivo.