Several studies have demonstrated that proteasome activity decreases whereas protein oxidation increases with aging in various tissues. However, no studies are available correlating both parameters directly comparing different tissues of one organism. Therefore, we determined whether there is an age-related change in proteasome activity and protein oxidation in heart, lung, liver, kidney and skeletal muscle samples of 6-, 10-, 18- and 26-month-old rats. There was a significant age-related increase in protein carbonyls at 18 and 26 months compared to young rats. Thereby, protein carbonyl formation was rather due to a general than a specific protein carbonylation as shown by immunblot studies. The highest increase in protein carbonyl formation was found in liver, lung and kidney samples. Proteasome activity decreased significantly with age in lung and liver samples. Proteasome activity in liver and lung decreased by factor five compared to young rats. Strong correlations between proteasome activity and protein oxidation were found in liver and lung, whereas in other tissues only a trend was found. These results demonstrate that the increase in protein oxidation and the decline in proteasome activity are correlating. Further studies are needed to determine the mechanisms which cause organ-specific aging-rates and their consequences.