The actions of potassium nitrate, thiourea, ethephon, sodium azide, and hydrogen peroxide on germination of Chenopodium album L., Rumex crispus L. and Avena fatua L. were investigated at constant and alternating temperatures. Ethephon and azide slightly stimulated germination of C. album seeds at 20°C while nitrate, thiourea and azide interacted positively with alternating temperatures (5°125°C, 8h/16h). Germination (expressed in normal equivalent deviates) increased linearly with the logarithm of chemical dose up to an optimal concentration. When azide was pre-applied for 24 hours the optimum concentration was 100 times greater than that when it was applied during the test. Fluctuating temperatures broke dormancy of R. crispus seeds and the chemicals only inhibited germination at high doses. Exposure to light when pre-applying azide and peroxide increased germination of C. alhum and R. crispus, but did not affect the actions of the chemicals. The compounds tested had little or no effect in A. fatua at constant or alternating regimes with a mean temperature over 10°C, but dormancy was relieved by sodium azide and potassium nitrate at low temperatures (3-10°C). These results demonstrate that the actions of various dormancy-relieving chemicals may depend on interactions with temperature treatments which are themselves likely to break dormancy, whether it is a low constant temperature in A. fatua or alternating temperatures in C. album.