Electric and magnetic fields (EMFs) associated with the production, transmission, and use of electricity are ubiquitous in industrialized societies. These fields are predominantly of low frequency (50/60 Hz) and are generally of low intensity. Review of the epidemiological evidence shows that the association between exposure to EMFs and cancer is weak and inconsistent, and generally fails to show a dose-response relationship. Moreover, in view of the methodological problems of these epidemiological studies, animal and laboratory studies are urgently needed to determine whether EMFs could be initiators and/or promoters of cancers. The objective of the present study was to determine whether chronic exposure to 60 Hz linear (single axis) sinusoidal, continuous-wave magnetic fields (MFs) of different intensities might increase the risk of leukemia and solid tumor development in rodents born and raised under these fields. Five groups of 50 female F344 rats were exposed for 20 h/day to 60 Hz MFs at intensities of <0.02 (sham controls), 2, 20, 200, and 2000 microT. Full body exposure to the different fields was administered for 104 wk starting from the prenatal period (2 days before birth) and continuing during lactation and weaning until late adult life. Body weight, survival, and clinical observations were evaluated in all groups of animals during in-life exposure. Necropsy was performed on all exposed and control animals that died, were found moribund, or were killed at termination of the study. To preserve and demonstrate the absence of any experimental bias, all clinical observations and pathological evaluations were conducted under "blinded" conditions. Fifty organs and tissues were evaluated in each animal, with special attention to the incidence of mononuclear cell leukemia, brain tumors, and mammary tumors. The findings from this chronic carcinogenicity study demonstrate that, under our defined experimental conditions, exposure to 60 Hz linear (single axis) sinusoidal, continuous wave MFs did not affect animal survival, solid tumor, or mononuclear cell leukemia development in female F344 rats. No statistically significant, consistent, positive dose-related trends with the number of tumor-bearing animals per study group could be attributed to MF exposure.