Electric and magnetic fields (EMF) are associated with the production, transmission, and use of electricity; thus, the potential for human exposure is high. These electric and magnetic fields are predominantly of low frequency (60 Hz in the United States and 50 Hz in Europe) and generally of low intensity. Epidemiology studies have suggested a potential for increased breast cancer, brain cancer, and leukemia rates with increasing magnetic field exposure. Therefore, given the widespread exposure to low-intensity, 60-Hz magnetic fields in industrialized societies, standard toxicology studies and long-term carcinogenesis studies were conducted using traditional rodent models. Male and female F344/N rats and B6C3F1mice were exposed to 60-Hz magnetic fields by whole-body exposure for 2 years. 2-YEAR STUDY IN RATS: Groups of 100 male and 100 female rats were exposed to 60-Hz magnetic fields at intensities of 0.02, 2, or 10 G for 18.5 hours per day, 7 days per week, for 106 weeks. Groups of 100 male and 100 female control rats were housed in the same exposure chambers without applied magnetic fields. Additional groups of 100 male and 100 female rats were intermittently exposed (1 hour on and 1 hour off) to a 10 G 60-Hz field 18.5 hours per day, 7 days per week, for 106 weeks. The highest field intensity (10 G) is approximately 5,000-fold greater than what was considered high intensity for homes in epidemiology studies in humans. Survival and Body Weights: Survival and mean body weights of exposed groups of male and female rats was similar to those of the control groups. Pathology Findings: The incidences of thyroid gland C-cell adenoma and carcinoma in 0.02 G male rats, adenoma in 2 G males, and adenoma or carcinoma (combined) in 0.02 and 2 G males were significantly greater than in the control group. The incidence of mononuclear cell leukemia in males in the 10 G intermittent group was significantly less than in the control group. 2-YEAR STUDY IN MICE: Groups of 100 male and 100 female mice were exposed to 60-Hz magnetic fields at intensities of 0.02, 2, or 10 G for 18.5 hours per day, 7 days per week, for 106 weeks. Groups of 100 male and 100 female control mice were housed in the same exposure chambers without applied magnetic fields. Additional groups of 100 male and 100 female mice were intermittently exposed (1 hour on and 1 hour off) to a 10 G 60-Hz field 18.5 hours per day, 7 days per week, for 106 weeks. Survival and Body Weights: Survival of male mice exposed to 10 G was significantly less than that of control mice after 2 years; survival of all other exposed groups of mice was similar to that of control mice. Mean body weights of exposed groups of male and female mice were similar to those of the control groups throughout the study. Pathology Findings: The incidences of alveolar/bronchiolar adenoma were significantly decreased in 0.02 and 2 G male mice and 2 G female mice relative to the control groups; the incidences of alveolar/bronchiolar adenoma or carcinoma (combined) were significantly less in males and females exposed to 2 G than in the control groups. In female mice, the incidence of malignant lymphoma in the 10 G intermittent group was significantly less than in the controls. CONCLUSIONS: Under the conditions of these 2-year whole-body exposure studies, there was equivocal evidence of carcinogenic activity of 60-Hz magnetic fields in male F344/N rats based on increased incidences of thyroid gland C-cell neoplasms in the 0.02 and 2G groups. There was no evidence of carcinogenic activity in female F344/N rats or male or female B6C3F1 mice exposed to 0.02, 2, or 10 G, or 10 G intermittent 60-Hz magnetic fields. In exposed rats and mice there were no increased incidences of neoplasms at sites for which epidemiology studies have suggested an association with magnetic fields (brain, mammary gland, leukemia).