This study has examined the effect of dose rate or fractionation on the carcinogenic effects of fission neutrons with emphasis on the dose range below 50 rad. The induction of lung adenocarcinomas, mammary adenocarcinomas, and ovarian tumors in female BALB/c mice was examined after whole-body neutron irradiation delivered at a high dose rate as a single exposure, or delivered as two equal fractions separated by intervals of 24 hr or 30 days and compared these effects to those after neutron irradiation at low dose rates. The dose responses for ovarian tumorigenesis after the split-dose fractionation regimen were similar to that observed after single high-dose-rate neutron exposure. However, lowering the dose rate reduced the incidence over the dose range of 0-50 rad. For lung and mammary tumors the results were more complex. These data suggest that fractionation and dose-rate effects are different for different tumor types presumably because of the different mechanisms of tumorigenesis that may be involved.