The influence of 46 herbicides, 17 fungicides and 15 insecticides on transformations of urea nitrogen in soil was studied by determining the effects of different amounts of each pesticide on the amounts of urea hydrolyzed and the amounts of nitrite and nitrate produced when samples of two coarse-textured and two fine-textured soils were incubated aerobically at 20(DEGREES)C for various times after treatment with urea. The pesticides tested are commonly used for control of weedy plant species, plant pathogens and soil insects in a range of cropping situations;When the 46 herbicides tested were applied at the rate of 5 (mu)g g('-1) soil, they had little or no effect on urea hydrolysis or nitrification of urea nitrogen in the four soils. When they were applied at the rate of 50 (mu)g g('-1) soil, only dinoseb retarded urea hydrolysis in all four of the soils, but all except siduron retarded nitrification of urea nitrogen in one of the coarse-textured soils and ten (2,4-D amine, 2,4-D ester, propanil, chlorpropham, alachlor, amitrole, dinoseb, propham, fenoxaprop ethyl and tridphane) retarded nitrification of urea nitrogen in all four soils;When the 17 fungicides tested were applied at the rate of 1 (mu)g g('-1) soil, only maneb retarded hydrolysis of urea and only terrazole retarded nitrification of urea nitrogen in the four soils used. When they were applied at the rate of 50 (mu)g g('-1) soil, most of them retarded urea hydrolysis in the two coarse-textured soils and all except anilazine, benomyl, chloroneb, fenarimol, iprodione and PCNB markedly retarded nitrification of urea nitrogen in all four of the soils;When the 15 insecticides tested were applied at the rate of 5 (mu)g g('-1) soil, none of them retarded urea hydrolysis in the four soils used, but five (carbaryl, lindane, trimethacarb, diazinon and fenitrothion) retarded nitrification of urea nitrogen in the two coarse-textured soils. When the insecticides were applied at the rate of 50 (mu)g g('-1) soil, six of them (ethoprop, fenitrothion, fonofos, isofenfos, carbaryl and lindane) retarded urea hydrolysis in the two coarse-textured soils and all of them retarded nitrification of urea nitrogen in these soils, but only fenitrothion and lindane markedly retarded nitrification of urea nitrogen in all four of the soils used.