Lipid peroxidation contributes to the damage of the sperm plasma membrane. In different species, dietary supplementation with antioxidants has been shown to improve semen quality. Therefore, we tested effects of dietary supplementation with antioxidants and l-carnitin on semen quality in Shetland pony stallions (n=6). Semen was collected twice a week over a time period of 16 weeks. From weeks 5 to 12, a special diet for stallions containing a variety of antioxidants (STALLION, Pavo Pferdenahrung GmbH, Goch, Germany; tocopherol 300 mg/day; ascorbic acid 300 mg/day; l-carnitin 4000 mg/day; folic acid 12 mg/day) was added to the basal diet (hay, mineral supplements, water). Ejaculates were evaluated for total sperm count, semen motility (percentage of totally and progressively motile spermatozoa, longevity for 24 h at 5 degrees C) and membrane integrity (SYBR-14/PI staining): All values given are means+/-S.E.M. No changes in motility, progressive motility and membrane integrity or semen longevity for 24 h were detected. A slight but significant reduction of morphologically abnormal spermatozoa was found (weeks 1-4: 43.7+/-7.1%; weeks 13-16: 39.4+/-7.2%, p<0.05). Results show that a supplementary diet with antioxidants in the given concentration and duration does not result in pronounced effects on semen quality of stallions. It is therefore questionable to support stallions with dietary antioxidants as long as they receive an adequately balanced basal diet.