Soil uptake, translocation, and metabolism of simazine (2-chloro-4,6-bis(ethylamino)-s-triazine) by tolerant poplar clone Populus × euramericana I-45/51, a section Aigeiros hybrid, and intolerant clone P. maximowiczii × P. trichocarpa Kingston, a section Tacamahaca hybrid were compared. Concentrations of simazine in the two clones were not different 48 h and 1 week after application. However, the presence of 2-chloro-4-amino-6-ethylamino-s-triazine (G-28279), a less toxic simazine metabolite, was detected in tolerant clone I-45/51 after 12 h and it increased steadily through 1 week after simazine application. In contrast, no trace of G-28279 was found in susceptible clone, Kingston.