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The present study was carried out to clarify the effect of oral administration of proanthocyanidin (PA) on radical-scavenging activity in muscle and plasma using electron spin resonance (ESR). Eight-week-old male Wistar rats were orally administered with 3 doses per day of 1 mL of 0.05% (PA0.05), 0.5% (PA0.5) or 5% (PA5) PA for 1 week. Control animals received the same volume of distilled water. We also examined the effect of a single dose of 0.5% PA. Blood and muscle were collected from rats 1 h after the final administration. Scavenging activity against superoxide anions in the plasma and m. soleus (Sol), m. plantaris (Pla), deep and surface areas of the m. gastrocnemius (GasD and GasS, respectively) and myocardium (Hrt) was determined using ESR with the spin trap, 5,5-dimethyl-1-pyrroline-N-oxide The scavenging activity in plasma for all groups given PA was 34%–44% higher than the control (p < 0.05). The scavenging activity in Hrt, Sol and GasD increased by up to 50% compared with the control and tended to increase depending on the dose of PA (p < 0.05). The impact of a single dose of PA was undetectable in all tissues. These results suggested that 1 week of oral PA improves the radical-scavenging activity in both plasma and muscle, especially in highly oxidative muscle. A single dose of PA was insufficient to improve the antioxidative capacity of muscle tissues. [ABSTRACT FROM AUTHOR]