The experiment was performed in support of a Japanese initiative to investigate the biological effects of irradiation from residual neutron-activated radioactivity that resulted from the A-bombing. Radionuclide Mn (T = 2.58 h) is one of the main neutron-activated emitters during the first hours after neutron activation of soil dust particles. In our previous studies (2016-2017) related to irradiation of male Wistar rats after dispersion of MnO powder, the internal doses in rats were found to be very inhomogeneous: distribution of doses among different organs ranged from 1.3 Gy in small intestine to less than 0.0015 Gy in some of the other organs. Internal doses in the lungs ranged from 0.03 to 0.1 Gy. The essential pathological changes were found in lung tissue of rats despite a low level of irradiation. In the present study, the dosimetry investigations were extended: internal doses in experimental mice and rats were estimated for various activity levels of dispersed neutron-activated MnO powder. The following findings were noted: (a) internal radiation doses in mice were several times higher in comparison with rats under similar conditions of exposure to MnO powder. (b) When 2.74 × 10 Bq of MnO powder was dispersed over mice, doses of internal irradiation ranged from 0.81 to 4.5 Gy in the gastrointestinal tract (small intestine, stomach, large intestine), from 0.096 to 0.14 Gy in lungs, and doses in skin and eyes ranged from 0.29 to 0.42 Gy and from 0.12 to 0.16 Gy, respectively. Internal radiation doses in other organs of mice were much lower. (c) Internal radiation doses were significantly lower in organs of rats with the same activity of exposure to MnO powder (2.74 × 10 Bq): 0.09, 0.17, 0.29, and 0.025 Gy in stomach, small intestine, large intestine, and lungs, respectively. (d) Doses of internal irradiation in organs of rats and mice were two to four times higher when they were exposed to 8.0 × 10 Bq of MnO (in comparison with exposure to 2.74 × 10 Bq of MnO). (e) Internal radiation doses in organs of mice were 7-14 times lower with the lowest MnO amount (8.0 × 10 Bq) in comparison with the highest amount, 8.0 × 10 Bq, of dispersed MnO powder. The data obtained will be used for interpretation of biological effects in experimental mice and rats that result from dispersion of various levels of neutron-activated MnO powder, which is the subject of separate studies.