The objectives of this study were to assess oxidant damage during and after spaceflight and to compare the results against bed rest with 6° head-down tilt. We measured the urinary excretion of the F₂ isoprostane, 8-iso-prostaglandin (PG) F₂, and 8-oxo-7,8-dihydro-2 deoxyguanosine (8-OH DG) before, during, and after long-duration spaceflight (4-9 mo) on the Russian space station MIR, short-duration spaceflight on the shuttle, and 17 days of bed rest. Sample collections on MIR were obtained between 88 and 186 days in orbit. 8-iso-PGF₂ and 8-OH DG are markers for oxidative damage to membrane lipids and DNA, respectively. Data are mean ± SE. On MIR, isoprostane levels were decreased inflight (96.9 ± 11.6 vs. 76.7 ± 14.9 ng · kg¹ · day¹, P < 0.05, n = 6) due to decreased dietary intake secondary to impaired thermoregulation. Isoprostane excretion was increased postflight (245.7 ± 55.8 ng · kg¹ · day¹, P < 0.01). 8-OH DG excretion was unchanged with spaceflight and increased postflight (269 ± 84 vs 442 ± 180 ng · kg¹ · day¹, P < 0.05). On the shuttle, 8-OH DG excretion was unchanged in- and postflight, but 8-iso-PGF₂ excretion was decreased inflight (15.6 ± 4.3 vs 8.0 ± 2.7 ng · kg¹ · day¹, P < 0.05). No changes were found with bed rest, but 8-iso-PGF₂ was increased during the recovery phase (48.9 ± 23.0 vs 65.4 ± 28.3 ng · kg¹ · day¹, P < 0.05). The changes in isoprostane production were attributed to decreased production of oxygen radicals from the electron transport chain due to the reduced energy intake inflight. The postflight increases in the excretion of the products of oxidative damage were attributed to a combination of an increase in metabolic activity and the loss of some host antioxidant defenses inflight. We conclude that 1) oxidative damage was decreased inflight, and 2) oxidative damage was increased postflight.