Administration of 1-methyl-4-piperidyl-bis(p-chlorophenoxy) acetate (SaH 42-348) at a dietary concentration of 0.15% for 3 weeks, increased the activity of catalase in both liver and kidney of male wild type (Cs-a strain) mice. A marked increase in the activity of short chain carnitine acyltransferase in the liver was also noted. By electron microscopy, a remarkable increase in the number of peroxisomes (microbodies) was noted in the liver cells. A mild to moderate increase in number of peroxisomes in proximal convoluted tubular epithelium of the kidney was also seen. These studies demonstrate that SaH 42-348 is a potent hepatic peroxisome proliferator.