The levels of peroxisomal enzyme activities in cultured C3H/10T1/2 CL8 cells and in chemically transformed C3H/10T1/2 MCA16 cells were studied after treatment with tiadenol and niadenate, two hypolipidemic drugs which are both carcinogenic and cause peroxisome proliferation in vivo. Administration of these peroxisome proliferators to the cells resulted in large increases in specific palmitoyl-CoA hydrolase, carnitine acetyl-transferase, and catalase activities. A reproducible induction of cyanide-insensitive palmitoyl-CoA oxidative activity was observed 5 and 9 days after initiation of tiadenol treatment. Basal activity was 0.16 nmole/min/mg protein compared to 0.95 nmole/min/mg protein in cells treated with 18 microM tiadenol (cytotoxicity of about 25%) for 9 days. The enzyme activities were more increased in the transformed MCA 16 cells than in the non-transformed cells and the order of increase in enzyme activities was: niadenate greater than tiadenol. In non-transformed cells, the specific activity of palmitoyl-CoA hydrolase was enhanced approximately 2.1-fold within 4 days after tiadenol treatment. During this time period the enzyme activity was constant in untreated cells, but decreased during longer incubation periods. The enhancement of palmitoyl-CoA hydrolase, carnitine acetyl-transferase and catalase activities was dose-related over a concentration range of 2 to 20 microM tiadenol, depending on the enzyme assayed. Tiadenol concentrations above 10 microM were increasingly cytotoxic, while 18 microM niadenate had no toxic effect on the C3H/10T1/2 C18 cells. Moreover, the stimulation of the 3 enzyme activities by the peroxisome proliferators were inhibited by cycloheximide. Neither of the two cell lines contained any appreciable urate oxidase activity. The responses of these cells to hypolipidemic drugs show that they constitute a useful system for studies on the role of peroxisomes in lipid metabolism and the relationship between hypolipidemic activity and carcinogenic potential of these drugs.