2-methylresorcinol (2-MR) was administered to groups of 40 male and 35 female Sprague-Dawley rats by admixture with diets at levels of 0.1, 0.4, and 1.5% for periods up to 6 months. Methemoglobin levels were determined after 6 weeks. After 90 days 10 animals/sex/group were killed for clinical pathological and histopathologic determinations. The 25 remaining females and 20 males per group were utilized in teratology and dominant lethal studies presented in Part II (T.A. Re, R.F. Loehr, S.C. Rodriguez, D. E. Rodwell, C.M. Burnett, 1986, Fundam. Appl. Toxicol. 7, 293-298). Ten additional males were killed after 6 months of exposure for additional clinical pathologic determinations and gross pathologic observations. The 20 males/group used in the dominant lethal study (Part II) were also killed after 6 months to serve as a comparison recovery group (gross examination of organs). Feeding 2-MR at a level of 1.5% in the diet was associated with a significant reduction in body weight gains. Females fed at a level of 0.4% also weighed significantly less than the control. No pathological effects were noted after either 90 or 180 days of feeding.