One-year-old and 3-month-old black walnut (Juglans nigra L.) and yellowpoplar (Liriodendron tulipifera L.) seedlings were treated with ring-labeled 14C-simazine [2-chloro-4,6-bis(ethylamino)-s-triazine] in nutrient culture. Simazine uptake was measured and the levels of simazine and its degradation products monitored at 3, 6, and 9 days following treatment for 1-year-old seedlings and at 3, 8, and 13 days for 3-month-old seedlings. Concentration of simazine and the phytotoxic degradation product monodealkylated simazine [2-chloro-4-amino-6-(ethylamino)-s-triazine] were significantly higher in yellowpoplar than in black walnut leaves. For 1-year-old seedlings, higher concentrations of phytotoxic compounds in yellowpoplar than in black walnut were attributed to greater simazine degradation by black walnut. For 3-month-old seedlings, higher concentrations in yellowpoplar were attributed to greater simazine uptake by yellowpoplar and greater degradation in black walnut. Simazine degradation proceeded rapidly in both species and N-dealkylation was a major pathway. Monodealkylated simazine and 2-chloro-4,6-diamino-s-triazine were isolated from various tissues of black walnut and yellowpoplar. Hydroxysimazine [2-hydroxy-4,6-bis(ethylamino)-s-triazine] was found in yellowpoplar roots but not in any black walnut extracts. Two other degradation products were isolated but not identified.