This study was intended to explore the oxidative status of broilers under cyclic heat stress (HS) as modulated by supplementation of mannan-oligosaccharides (MOS) and a probiotic mixture (PM). Two hundred fifty 1-d-old chicks were randomly divided into 5 groups. From d 22, birds were either kept in a thermoneutral zone (TN; 26.7°C for 24 h/d) or subjected to HS (35 ± 1.1°C and 75 ± 5% RH for 8 h/d from 1000 to 1800 h) to the conclusion of the study on d 42. Birds were fed either a corn-based diet (TN and HS groups) or the same diet supplemented with 0.5% MOS (HS-MOS group), 0.1% PM (HS-PM group), or their combination as a synbiotic (SYN; HS-SYN group). On d 42, birds were killed by cervical dislocation to collect serum for the determination of total oxidants, total antioxidants, paraoxonase, arylesterase, ceruloplasmin, aspartate aminotransferase, alanine aminotransferase, and trace minerals. Heat stress increased (P < 0.05) total oxidants and total antioxidants and decreased (P < 0.05) paraoxonase and arylesterase, with no change in ceruloplasmin, aspartate aminotransferase, and alanine aminotransferase activities. Dietary supplementation decreased (P < 0.05) total oxidants and total antioxidants, with no effect on the activities of other enzymes. Heat stress did not influence serum copper, zinc, and manganese concentrations of birds when compared with those in the TN group. However, MOS increased (P < 0.05) concentrations of all the trace minerals, whereas SYN increased (P < 0.05) concentrations of only zinc and copper. It was concluded that MOS or PM supplementation, alone or as a SYN, may reduce some of the detrimental effects of HS, whereas MOS alone or as a SYN may improve the absorption of trace minerals.