The persistence of hormone-type herbicides in tissues of susceptible crop plants was studied under growth-chamber and field conditions. The susceptible plants selected were tomatoes (Lycopersicon esculentum Mill.), soybeans [Glycine max (L.) Merr.], and turnips (Brassica rapa L.). Foliage of each species was exposed to herbicides at levels that represented spray drift situations, and the decline of residues was investigated through subsequent chemical analysis. The disappearance of 2,4-D [(2,4-dichlorophenoxy)-acetic acid], 2,4-DB [4-(2,4-dichlorophenoxy)butyric acid], mecoprop {2-[(4-chloro-o-tolyl)oxy]propionic acid}, 2,4,5-T [(2,4,5-trichlorophenoxy)acetic acid], and dicamba (3,6-dichloro-o-anisic acid) was almost linear when concentration data were plotted against the logarithm of time. The results indicate that for practical purposes a kinetic rate equation [conc. = a – b log (time)] would describe the residue decline.