The persistence of the herbicides cyanazine, metribuzin, and atrazine was studied at different controlled temperature and moisture contents in Regina Heavy Clay soil. Degradation of the herbicides approximated first-order kinetics at temperatures in the range 5–30 °C and at moistures of above 20%. The breakdown of cyanazine was faster than metribuzin, which, in turn, was more rapid than atrazine. The breakdown of the three herbicides under field conditions, following June applications of 1.0 kg ha−1, was measured. Over 90% of the applied cyanazine had dissipated within 56 d, while the metribuzin and atrazine were more persistent, with approximately 30% of the initial treatment remaining at the end of the growing season. In all cases, there was no leaching of the chemicals below 10 cm. The laboratory data were used in conjunction with meteorological records in a computer program to simulate the dissipation pattern for the herbicides. There was good agreement between the observed and predicted residue levels for cyanazine, and a reasonable agreement for atrazine. For metribuzin, the model underestimated the rate of loss. Key words: Herbicides, atrazine, cyanazine, metribuzin, predicted field persistence