Female rats were implanted with mammary adenocarcinoma tissue and 25 days later exposed to 2,000-Hz magnetic fields 1 h a day for 9 days. Analysis showed that tumor weights and the levels of several reproductive hormones were not significantly different between treated and control animals. Other studies with a similar design have also reported no significant effects from magnetic field exposure. However, vaginal smear data from all animals revealed that handling and restraint stress may have confounded the detection of subtle magnetic field effects.