The effect of temperature and light after spraying on the activity of flumetsulam and metosulam when applied to seedlings of Raphanus raphanistrum was evaluated under controlled environments. Flumetsulam and metosulam were applied at 0.01–3 times the recommended doses and the plants were subjected to a range of temperatures after spraying. Herbicide activity was estimated from dose–response curves of fresh weight. Varying the temperature after spraying from 1 to 20 °C increased the activity of flumetsulam and metosulam, as determined by comparison of ED50, by a factor of 97 and 7 respectively. Large increases in herbicide activity occurred in the 1–5 °C range and smaller but significant increases at temperatures greater than 5 °C. No significant differences in the activity of the herbicides were found when the plants were subjected to light or dark conditions at 5 °C after spraying. The influence of temperature on activity may lead to opportunities for rate adjustment of flumetsulam and metosulam based on the temperature prevailing around the time of spraying.