OBJECTIVE To establish reference intervals for serum reactive oxygen metabolites (ROMs), biological antioxidant potential (BAP), and oxidative stress index (OSi) in adult rams by use of controlled preanalytic and analytic procedures. ANIMALS 123 healthy 1- to 4-year-old rams of 2 Greek breeds (Chios [n = 62] and Florina [61]). PROCEDURES 4 hours after rams were fed, a blood sample was obtained from each ram, and serum was harvested. Concentrations of ROMs and BAP were measured colorimetrically on a spectrophotometric analyzer. The OSi was calculated as ROMs concentration divided by BAP concentration. Combined and breed-specific reference intervals were calculated by use of nonparametric and robust methods, respectively. Reference intervals were defined as the 2.5th to 97.5th percentiles. RESULTS Reference intervals for ROMs, BAP, and OSi for all rams combined were 65 to 109 Carratelli units, 2,364 to 4,491 μmol/L, and 18.2 to 43.0 Carratelli units/(mmol/L), respectively. Reference intervals of Chios rams for ROMs, BAP, and OSi were 56 to 113 Carratelli units, 2,234 to 4,290 μmol/L, and 12.9 to 38.4 Carratelli units/(mmol/L), respectively. Reference intervals of Florina rams for ROMs, BAP, and OSi were 68 to 111 Carratelli units, 2,337 to 4,363 μmol/L, and 14.1 to 38.1 Carratelli units/(mmol/L), respectively. CONCLUSIONS AND CLINICAL RELEVANCE Reference intervals calculated in this study can be used as a guide for the interpretation of ROMs, BAP, and OSi results in rams and, under appropriate conditions, can be adopted for use by veterinary laboratories.