Reaction rate and dose rate distributions due to streaming o f neutrons and gamma rays were measured in the nearly cubical cavity and in the ducts leading to the cavity at the fast reactor YAYOI. The experimental configuration was arranged in a way that streaming through the ducts occurred only after scattering in the cavity. This was done by making the duct axes perpendicular to the source radiation from the reactor core. The spectrum of the source was modified from fast to thermal by putting moderators in the beam hole of the core assembly. The spatial distribution of the neutron reaction rates and gamma-ray dose rates in the cavity is almost flat except for a small decrease at the wall. The cadmium ratios at the duct inlets and the attenuation profiles in the ducts are almost identical irrespective of the spectrum of neutrons entering the cavity. The two-dimensional discrete ordinates transport calculation by TWOTRAN-II has shown that although the ray effect is observed in the fast neutron flux distribution in the cavity, the calculated results agree rather well with the experimental distributions in the cavity and with the streaming in the ducts with regard to the results measured with the l/v type neutron detectors. Also, for the gamma rays the agreement is rather good between the calculation and the experiment.