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A magnetic field exposure laboratory has been constructed to support National Toxicology Program studies for the evaluation of the toxicity and carcinogenicity of pure, linearly polarized, 60 Hz magnetic fields in rodents. This dual corridor, controlled access facility can support the simultaneous exposure of 1200 rats and 1200 mice. The facility contains fully redundant electrical and environmental control systems and was constructed using non-metallic materials to maintain low levels of background (ambient), stray, and cross-talk magnetic fields. The exposure module design provides for large uniform exposure volumes with good control of stray and cross-talk fields, while allowing the use of roll-around cage racks for simplified animal husbandry. Stray fields and cross-talk have been further reduced by the inclusion of "steering coils" in each exposure module. Ambient 60 Hz fields (less cross-talk) in all exposure rooms are <0.1 microT (1 mG), and static magnetic fields have been mapped extensively. Magnetic field strength, waveform, temperature, relative humidity, light intensity, noise level, vibration, and air flow in all animal holding areas are tightly regulated, and are monitored continuously during all studies. Field uniformity in the animal exposure volumes is better than -/+l0%; a systematic program of cage, rack, and room rotation controls for possible positional effects within the exposure system. Magnetic fields are turned on and off over multiple cycles to prevent the induction of transients associated with abrupt field level changes. Total harmonic distortion is <3% at all field strengths. The facility has been used to study magnetic field bioeffects in rodent model systems in experiments ranging in duration from 8 weeks to 2 years.