The degradation of peroxisomal and nonperoxisomal proteins by endoproteases of purified peroxisomes from senescent pea (Pisum sativum L.) leaves has been investigated. In our experimental conditions, most peroxisomal proteins were endoproteolytically degraded. This cleavage was prevented, to some extent, by incubation with 2 mM phenylmethylsulfonylfluoride, an inhibitor of serine proteinases. The peroxisomal enzymes glycolate oxidase (EC 1.1.3.1), catalase (EC 1.11.1.6) and glucose-6-phosphate dehydrogenase (EC 1.1. 1.49) were susceptible to proteolytic degradation by peroxisomal endoproteases, whereas peroxisomal manganese superoxide dismutase (EC 1.15.1.1) was not. Ribulose-1,5-bisphosphate carboxylase/oxygenase (EC 4.1.1.39) from spinach and urease (EC 3.5. 1.5) from jack bean were strongly degraded in the presence of peroxisomal matrices. These results indicate that proteases from plant peroxisomes might play an important role in the turnover of peroxisomal proteins during senescence, as well as in the turnover of proteins located in other cell compartments during advanced stages of senescence. On the other hand, our data show that peroxisomal endoproteases could potentially carry out the partial proteolysis which results in the irreversible conversion of xanthine dehydrogenase into the superoxide-generating xanthine oxidase (EC 1. 1.3.22). This suggests a possible involvement of the peroxisomal endoproteases in a regulated modification of proteins.