Total body nitrogen (TBN) was determined in 16 rat carcasses ranging in weight from 55 to 550 g, by non-destructive 14 MeV neutron activation analysis (NAA). The rat carcasses were subsequently analysed for TBN by Kjeldahl digestion, for total body water (TBW) by loss of weight after freeze-drying and for body fat by adiabatic bomb calorimetry after subtraction of protein energy. TBN results by the two methods were in good agreement, the precision by NAA (coefficient of variation = 1.5%) being superior to that by chemical analysis (coefficient of variation = 2.8%). Body fat calculated by difference from a combination of measured TBW and NAA data agreed closely with bomb calorimetry measurements. The use of indirect estimates of TBW to determine fat gave poor results. A group of four growing rats was analysed sequentially by NAA four times in 2 weeks. The maximum total radiation dose received by each animal was less than 50 rem (less than 500 mSv) and no significant differences in growth rate were observed compared with non-irradiated control groups. 14 MeV NAA in vivo can be conducted with sufficient precision to measure 0.14 g TBN changes in growing rats at 2 weeks post-weaning.