Male rats were given 0.25% CPIB (ethyl chlorophenoxyisobutyrate) for 4 weeks at which time their liver cells showed the typical increase in number of microbodies (peroxisomes). During the first week after withdrawal of CPIB and simultaneous injection of allylisopropylacetamide (AIA), an inhibitor of catalase synthesis, the microbody matrix, in peroxidase preparations, showed marked decrease in electron opacity. In liver cells of rats from which CPIB was withdrawn but which were not given AIA, there were interruption in the limiting membrane and almost total loss of microbody matrix within 3 days. Simultaneous with the loss of matrix content, the hepatic catalase activity decreased from an average 80 units/mg protein to less than 10 units/mg protein. In liver cells of Mastomys natalensis, a multimammate rodent, withdrawal of CPIB was associated with the frequent occurrence of bare microbody nucleoids situated free in the hyaloplasm. The ultrastructural observations and the associated decrease in catalase activity suggest that one means of disposal of microbodies is by rather rapid dissolution or leakage of their matrix enzymes into the surrounding hyaloplasm.