Gamma radiation of 3 megarads was applied to undiluted commercial formulations and aqueous solutions of 3-(3,4 dichlorophenyl)-1,1-dimethylurea (diuron), 1,1-dimethyl-(3-a,a,a-trifluoro-m-tolyl)-urea fluometuron), N,N-dimethyl-2,2-diphenylacetamide (diphenamid), 2,4-bis (isopropylamino)-6-(methylthio)-s-triazine (prometryne), dimethyl tetrachloroterephthalate (DCPA), a,a,a-trifluoro-2,6-dinitro-N,N-dipropyl-p-toluidine (trifluralin) and sodium chlorate. The effect of radiation on herbicidal activity was assessed with bioassays. Radiation did not affect the bioactivity of undiluted or 50% solutions of any of the tested herbicides but decreased appreciably the activity of 100-ppm solutions of fluometuron, diphenamid, and prometryne. Diluted 100 and 10-ppm solutions of trifluralin lost part of their activity during storage in plastic containers and, in addition, following irradiation. It was not clear whether the decrease in activity observed in the 100-ppm solution of diuron was due to irradiation or to storage. No reduction in bioactivity was produced following irradiation of 100 and 10-ppm solutions of DCPA or 1% and 10% solutions of sodium chlorate.