The effects of rainfall and temperature upon herbicidal activity of 2-chloro-N,N-diallylacetamide (CDAA), 2-chloro-4,6-bis-(ethylamino)-s-triazine (simazine), and 2-chloro-4-ethylamino-6-isopropylamino-s-triazine (atrazine), for the control of foxtail (Setaria spp.) were determined. CDAA spray was effective if applied to warm soil and if ½ inch of rain fell during the first week after application, unless a heavy rain fell immediately after application. The effectiveness of granules was not impaired by a heavy rain immediately after treatment or by a delay of 2 weeks before receiving ½ inch of rain. Simazine and atrazine sprays and granules were effective if 1 inch of rain fell within 15 days after application of simazine or if ½ to 1 inch fell within 15 to 20 days afterwards for atrazine. Mechanical incorporation aided in activation of simazine when rainfall was slightly below minimum required for activation.