Based on results of simulation and empirical studies conducted within the divisions of biometrics, center for drug evaluation and research, food and drug administration, and in collaboration with the national toxicology program, the center has recently changed the significance levels for testing positive linear trend in incidence rate for common and rare tumors, respectively, from 0.01 and 0.05 to 0.005 and 0.025. the overall false positive rate resulting from the use of this new rule in the tests for linear trend in a two-species-two-sex study is about 10%, the rate that is judged as the most appropriate in a regulatory setting by the center. this paper descries two of the studies.