This annual report is a compilation of the research activities and achievement in the National Institute of Radiological Sciences (NIRS) in Japan during the fiscal year 1991 (from April 1991 through March 1992). The research covers a wide range of radiological sciences from molecular to environmental studies and medicine including engineering. Topics consists of physics, chemistry, biomedical science, clinical research, and environmental sciences, covering a total of 69 titles. A list of publications by staff members, activities of research divisions, and organization chart of the NIRS are given in Appendix. (J.P.N.) 64 refs.