Based on usage of the parent compounds and studies of their dissipation in corn fields, atrazine (6-chloro-N-ethyl-N'-(1-methylethyl-1,3,5-triazine-2,4-diamine)), cyanazine (2-[[4-chloro-6-(ethylamino)-1,3,5-triazin-2-yl]amino]-2-methylpropionitrile), and simazine (6-chloro-N,N'diethyl-1,3,5-triazine-2,4-diamine) are thought to be the important contributors of desethylatrazine (6-chloro-N-(1-methylethyl)-1,3,5-triazine-2,4-diamine) or DEA and desisopropylatrazine(6-chloro-N-ethyl)-1,3,5-triazine-2,4-diamine) or DIA to ground water. Atrazine degrades to both DEA and DIA by dealkylation. DEA is transported through the unsaturated zone more readily than DIA because of the more rapid degradation of DIA in the shallow unsaturated zone. Both cyanazine and simazine degrade to DIA by dealkylation, and DIA may be an indicator of leaching and subsequent degradation from both parent compounds. Because cyanazine has an acid intermediate that is mobile in the unsaturated zone, it may be an important source for DIA transport to ground water. Based on a regional survey of ground water of the Midwest during the Spring and Summer of 1991, DEA occurs most frequently with detections of DEA (15.4%) > atrazine (14.7%) > DIA (4%) > simazine (0.7%) > cyanazine (0.3%) > propazine (no detections). The DEA to atrazine ratio (DAR) is an indicator of atrazine source and transport, with the lowest ratios indicating most rapid transport.