To investigate the effects of neutrophil activation during hemodialysis (HD), blood markers of oxygen free radical (OFR) activity were studied. Two groups of HD patients on standard cuprophane treatment were investigated after an overnight fast. In the first group (mean age 68 +/- 8 years; n = 6) vitamin supplementation was withdrawn two weeks prior to the study, whereas the second group (mean age 73 +/- 3 years; n = 7) continued their normal vitamin intake. The two control groups, one consisting of age-matched subjects (mean age 72 +/- 2 years; n = 21), the other of younger subjects (mean age 36 +/- 7 years; n = 11), were asked to cease vitamin supplementation two weeks before the study and to fast overnight before blood sampling. Serial blood and dialysate samples were collected during HD in the vitamin-deprived patient group, and a single blood sample was collected in the other three groups. Plasma concentrations of vitamin C (total and reduced form), vitamin E (alpha-tocopherol) and malondialdehyde (MDA) were determined with newly adopted and validated HPLC methods. Basal plasma vitamin C concentrations were lower among vitamin-deprived HD patients than among age-matched controls or vitamin-supplemented HD patients (22 +/- 6 microM versus 39 +/- 19 microM and 34 +/- 10 microM, respectively). During a 3-hour HD session, the mean decrease in total vitamin C was 40%. Basal alpha-tocopherol concentrations did not differ significantly between vitamin-deprived HD patients and vitamin-supplemented HD patients or age-matched controls (39 +/- 5 microM versus 40 +/- 11 microM and 38 +/- 6 microM, respectively), but were lower in younger controls (33 +/- 4 microM). No alpha-tocopherol was detected in the dialysate, and its plasma concentration did not change significantly during a single HD session. Basal plasma MDA concentrations were higher in vitamin-supplemented HD patients than in vitamin-deprived HD patients or age-matched controls (1.5 +/- 0.2 microM versus 0.9 +/- 0.2 microM and 1.1 +/- 0.2 microM, respectively). No MDA was detected in the dialysate, and its plasma concentration did not change significantly during a single HD session. Our results indicate an increased need of vitamin C supplementation in HD patients. The concentration of oxidized vitamin C seems to peak early during HD and may be of value as a marker of OFR production. alpha-tocopherol concentrations do not change during HD and do not differ from those in control subjects. MDA may increase over a longer period of time on dialysis, but does not change during a single HD treatment.