[en] Neutron activation analysis methods with short irradiation were developed to determine very low quantities of Th in human excreta. The quantities detected were in the range 0,02-1,6 μg per sample. Detection limits of 1-37 ng Th were achieved. The samples are irradiated at thermal neutron fluxes of about 0,5*10¹⁸m^-2s^-1 to 0,8*10¹⁸m^-2s^-1. Irradiation periods are 3-7 minutes for 233Th-detection and 5-20 minutes for 233Pa-detection. Activation products of 232Th, either 233Th or 233Pa, are then detected by gamma spectroscopy, 1. 233Th-detecdon - 86,5 keV, 92-96 keV (X-rays of Pa): Because of the short half-life of 233Th (T= 22,3 min), measurements have to be performed as soon as possible after irradiation. For a high sensitivity of Th-detection chemical separation of Th in the preparation of irradiation samples and special measurement techniques have to be employed: -Th is separated from excreta in two steps: by coprecipitation with Ca-oxalate and anion exchange. The chemical yield of Th is determined by 234Th-tracer. -The irradiation samples are placed in very thin polyethylene foils (approx. 30 mg per sample), thus keeping activities of the container at very low levels. -Optimisation of the nuclear counting system. For 233Th-gamma-detection a Low Energy Ge-Detector (LEGe detector) is used, which has extremely low efficiency for high energy gamma rays. A very fast ADC (with a fixed dead time of 800 ns) is used. A plexiglass absorber is introduced to eliminate the beta-radiation from the samples. 2. 233Pa-detection: 99-106 keV (X-rays of U) and 311,9 keV. For gamma-detection of 233Pa two short irradiation methods were found suitable: -Pa-A-method: the samples prepared for 233Th-counting are measured a few days later for 233Pa, usually for 2-4 hours. -Pa-B-method: Th is separated from the dissolved faecal ashes by coprecipitation with Ca-oxalate and irradiated 20 minutes. About three weeks later the irradiated samples are counted for about 16 hours. Results of measurements: 12 faecal ash samples from three Th-unexposed humans were analysed by using 2mTh- and 2mPa-A-methods. For 11 samples Th-excretion was found to be between 0,3 and 1,0 μg Th per day, one sample had 2,0 μg Th per day. In connection with an intercomparison project two urine samples from PTB (Physikalisch-Technische Bundesanstalt Braunschweig) were analysed by 233Th- and Pa-A-Methods. For the first sample 260 μg Th per litre urine was found, for the second sample 13,5 μg Th per litre urine. This compares quite well with the true values of Th-concentration given by PTB: 282 and 14,2 μg Th per litre urine. (author)