Studies regarding the environmental impact of engineered nanoparticles (ENPs) are hampered by the lack of tools to localize and quantify ENPs in water, sediments, soils, and organisms. Neutron activation of mineral ENPs offers the possibility of labeling ENPs in a way that avoids surface modification and permits both localization and quantification within a matrix or an organism. Time-course experiments in vivo also may be conducted with small organisms to study metabolism and exposure, two aspects currently lacking in ecotoxicological knowledge about ENPs. The present report explains some of the prerequisites and advantages of neutron activation as a tool for studying ENPs in environmental samples and ecologically relevant organisms, and it demonstrates the suitability of neutron activation for Ag, Co/Co3O4, and CeO2 nanoparticles. In a preliminary experiment with the earthworm Eisenia fetida, the dietary uptake and excretion of a Co nanopowder (average particle size, 4 nm; surface area, 59 m2/g) were studied. Cobalt ENPs were taken up to a high extent during 7 d of exposure (concentration ratios of 0.16-0.20 relative to the ENP concentration in horse manure) and were largely retained within the worms for a period of eight weeks, with less than 20% of absorbed ENPs being excreted. Following dissection of the worms, 60Co was detected in spermatogenic cells, cocoons, and blood using scintillation counting and autoradiography. The experimental opportunities that neutron activation of ENPs offer are discussed.