Long-term absorption of 14C-atrazine [2-chloro-4 - (ethylamino) - 6 - (isopropylamino) - s - triazine] by excised velvetleaf (Abutilon theophrasti Medic.) roots was investigated. A rapid, initial phase of absorption was followed by a long-term, linear phase of absorption, which resulted in atrazine accumulation in the tissue to concentrations above the external solution. This accumulation did not result from metabolism of the atrazine. Aging of the excised tissue appeared to be responsible for the accumulation. Dinitrophenol, but not cyanide, inhibited accumulation. Atrazine accumulation decreased as the temperature during absorption was decreased. The Q10 the absorption rate was ca. 2.5 between 15 and 25 C. Efflux of atrazine from the tissue occurred in two phases. At 25 C, loss of 80% of the atrazine occurred in 20 min. At 4C, the rate of this efflux was slower and the phase lasted longer. At both 4 and 25 C a second, slower phase of efflux occurred through at least 6 h and resulted in loss of equal amounts of atrazine at the two temperatures. These experiments show that, in addition to rapid absorption of atrazine to a passive equilibrium, excised velvetleaf roots accumulate atrazine by a metabolically dependent process over long periods of time.