The aim of this study was to examine the effect of a low-carbohydrate (L-CHO) diet and graded cycling exercise on the enzymatic and non-enzymatic blood antioxidant defence system in young eumenorrhoeic women. Seven healthy physical education students exercised incrementally until they were fatigued under four different phase-diet conditions of the menstrual cycle, i.e. twice either during the mid-follicular or the mid-luteal phase, in each case either after 3 days of eating a normal mixed diet (59% carbohydrate, 27% fat, 14% protein) or 3 days of eating an isoenergy L-CHO diet (5% carbohydrate, 52% fat, 43% protein). In venous blood samples obtained at rest, immediately post test and during recovery, the activity of antioxidant enzymes and concentrations of reduced glutathione and selenium were determined. Plasma samples were analysed for concentrations of malondialdehyde, vitamin E (alpha-tocopherol), uric acid and activity of creatine kinase. The 3 days of the L-CHO diet, which had been preceded by glycogen-depleting exercise, resulted in a stimulation of the blood antioxidant defence system in young eumenorrhoeic women both at rest and during the graded cycling exercise to maximal oxygen uptake. It seems justified to presume that higher daily doses of haem iron, selenium and alpha-tocopherol provided by the L-CHO diet contributed to the enhancement of catalase activity, the rise in plasma concentrations of alpha-tocopherol and selenium, which resulted in better protection of the cell membranes against damage from peroxides, as reflected by a limited release of creatine kinase into plasma. With the exception of the case of glutathione reductase, the phases of the menstrual cycle had only minor effects on the indices of the blood antioxidant defence system.