We studied the toxic effects of a Sarcocystis hirsuta cyst extract fed to mice. Degenerative changes were found in mice gavage-fed fresh, frozen, and heat-treated S. hirsuta cyst extract. There were increases in the levels of serum aspartate aminotransferase and alanine aminotransferase as well as hepatic and brain malondialdehyde (MDA) levels along with concomitant decreases in catalase (CAT) and superoxide dismutase (SOD) activities of mice receiving fresh and frozen S. hirsuta extracts. Gavage feeding of heat-treated S. hirsuta cyst extract had no effects on liver enzymes or brain MDA content, but the liver MDA level did increase. Mice in the heat-treated cyst group showed reduced CAT and SOD activities as well as increased hepatic MDA levels compared to those in the control group. These results indicate that an extract of S. hirsuta cyst can induce oxidative stress and hepatic injury, even after heat treatment.