[en] Major activities and events concerning YAYOI (a fast neutron source reactor at the Nuclear Engineering Research Laboratory of the University of Tokyo) during fiscal 1973 and 1974 are summarized. In this period, the nonstationary operation of the reactor was approved and started. Also the low temperature irradiation facility became available and various experiments were performed. Licensing preparations for pulse-operation and the overhaul of components were also undertaken during this period. The details of major events are listed followed by a list of the history of reactor operation. The records of decision makings and of technical meetings are also presented. The items of inspections, both compulsory and voluntary, are listed together with the date and the results. Other major items included in this report are; overhaul of components, maintenance and repair, detail of the expences for maintenance, and the list of related documentation. (Aoki, K.)