The erroneous and misleading conclusions that result when data obtained in vivo are plotted on an hours after treatment format when compared to time of day or time of circadian period format are illustrated. The mitotic index of the corneal epithelium and the amount of DNA synthetic activity in the tip of the tongue are the examples used. Mice were kept on a light-dark cycle with light from 0600 to 1800 h CST. Treatment with an i.p. injection of 0.2 ml of saline at 0500 h compared to treatment with saline at 1700 h resulted in no effect on either variable when the data were plotted on a time of day format, i.e., the data from the group which received saline at 0500 h and the group which received saline at 1700 h were very producible. However, when the same data were plotted on the hours after treatment format, the data were 180.degree. out of phase with each other. This resulted in many statistically significant differences between the 2 groups. These differences are artifactual when compared to the no effect or no perturbation situation seen when the data are plotted on the time of day format. [The results are discussed in reference to chemotherapeutic treatment of cancer.].