This study investigated the long-term fate of (3)H-ivermectin and its metabolites in aerobic sediment/water systems. Total radioactivity in sediment and water was measured by Liquid Scintillation Counting and Radio-HPLC was used for determination of transformation products. The test system, containing a cooling and a water trap, showed excellent applicability for studying environmental fate of (3)H-labelled compounds. A complete mass balance was obtained over the whole duration with total recoveries ranging from 91.3% to 98.8%. In water, a DT(50)-value of ivermectin of <6h and a DT(90)-value of 16.8 d were obtained. The DT(50)-value of ivermectin in the entire system was 127 d. Maximum distribution coefficients were calculated to K(d)=160 L kg(-1) and K(oc)=3550 L kg(-1). The fraction of bound residues increased to a maximum percentage of 30.4% after 188 d. In total about 31.3% of the initially applied ivermectin were transformed after 188 d. Two transformation products ivermectin monosaccharide and ivermectin aglycone, could be identified in samples from day 100 and 188, respectively. However, additional peaks which were observed in the radio-chromatograms indicate that several other transformation product are formed.