AbstractUnder field conditions, there was little loss of herbicidal activity following spring application of pronamide when the soil temperature remained below about 13°c, but under normal summer conditions loss was rapid (half‐life 2–4 weeks). The rate of loss was retarded when the surface soil became very dry. After autumn application, there was no change in activity during the winter months and assays on samples taken in the following spring showed that little leaching had taken place from the surface 5 cm. In laboratory studies, breakdown was shown to follow first‐order kinetics. Half‐lives at 10% soil moisture were 29 days at 23°c, 63 days at 15°c and 140 days at 8°c. At 23°c the half‐life was extended to 52 days when the soil moisture content was reduced by half.