The potential environmental toxicity of zero-valent iron nanoparticles (nZVI) and three types of nanosilver differing in average particle size from 1 to 20 nm was evaluated using seed germination tests with ryegrass, barley, and flax exposed to 0–5000 mg L−1 nZVI or 0–100 mg L−1 Ag. For nZVI, germination tests were conducted both in water and in two contrasting soils to test the impact of assumed differences in bioavailability of nanoparticles. Inhibitory effects were observed in aqueous suspensions at 250 mg L−1 for nZVI and 10 mg L−1 for Ag. Reduction in shoot growth was a more sensitive endpoint than germination percentage. Complete inhibition of germination was observed at 1000–2000 mg L−1 for nZVI. For Ag, complete inhibition was not achieved. The presence of soil had a modest influence on toxicity, and inhibitory effects were observed at 300 mg nZVI L−1 water in soil (equivalent to 1000 mg nZVI kg−1 soil). Complete inhibition was observed at 750 and 1500 mg L−1 in sandy soil for flax and ryegrass, respectively, while for barley 13% germination still occurred at 1500 mg L−1. In clay soil, inhibition was less pronounced. Our results indicate that nZVI at low concentrations can be used without detrimental effects on plants and thus be suitable for combined remediation where plants are involved. Silver nanoparticles inhibited seed germination at lower concentrations, but showed no clear size-dependant effects, and never completely impeded germination. Thus, seed germination tests seem less suited for estimation of environmental impact of Ag. © 2010 Wiley Periodicals, Inc. Environ Toxicol 2012.