AbstractPolychlorotrifluoroethylene (polyCTFE—primarily oligomers with 3–4 monomer units), a non‐flammable hydraulic fluid for aircraft, was given daily for 15 days by oral gavage to four Rhesus monkeys at a concentration of 0.725 g kg−1. The administered dose was at a level that had caused toxicity in rats. Steady‐state blood and liver concentrations reached were the same in both species. In monkeys, polyCTFE did not cause the electrolyte, serum protein, liver enzyme and anemic disturbances previously seen in rats. Liver sections taken at 15 days, analyzed for palmitoyl Co‐A beta‐oxidation rates or by electron microscopy, showed no significant indication of peroxisomal proliferation. An increased blood urea nitrogen (BUN) at 15 days was the only clinical pathological abnormality seen in both monkeys and rats. Previously unobserved effects were increased triglycerides and glyco