Results from chronotoxicological investigations with the plant protection chemical DNOC has been described and judged. It has been studied hyperthermia caused by i.v. application of 7.5 mg/kg body weight on rabbits to 6 different times, starting at 4 a.m. with 4-hours-intervals as well as the dose-effect relation in each case with 3 doses to the time of application at 12 a.m., 6 p.m., 12 p.m. and 6 a.m. and the renal DNOC excretion after a dose of 10 mg/kg body weight. There was found a time dependent rhythm in all experiments. Acrophase of hyperthermia is placed in the middle of the phase of activity of the animals at 12 p.m., however it depends on the dose. The results are showing, that the position of acrophase by constant environmental factor is neither a substance specific nor an animal specific constant.