The influence of 14 urease inhibitors, 28 nitrification inhibitors and 33 pesticides on denitrification of nitrate in soil was studied by determining the effects of different amounts of these compounds on the amounts of nitrate lost and the amounts of nitrite, N(,2)O and N(,2) produced when soil samples were incubated anaerobically after treatment with nitrate or with nitrate and organic carbon (as mannitol or glucose). The urease and nitrification inhibitors tested included those considered to be the most effective compounds currently available for retardation of urea hydrolysis and nitrification in soil. The pesticides tested included 20 herbicides, 7 insecticides and 6 fungicides;Only two urease inhibitors (2,5-dimethyl-1,4-benzoquinone and 2,6-dimethyl-1,4-benzoquinone) and one nitrification inhibitor (potassium azide) inhibited denitrification when applied at the rate of 50 (mu)g g('-1) soil. Two other urease inhibitors (catechol and sodium-4-chloromercuribenzoate) and six other nitrification inhibitors (sulfathiazole, potassium ethylxanthate, sodium isopropylxanthate, 4-nitrobenzotrichloride, sodium thiocarbonate and phenylmercuric acetate) retarded denitrification when applied at the rate of 50 (mu)g g('-1) soil to soils amended with mannitol. The other urease and nitrification inhibitors studied either had no significant effect on denitrification, or enhanced denitrification, when applied at rates up to 100 (mu)g g('-1) soil. Reports that nitrapyrin (N-Serve) and etridiazole (Dwell) inhibit denitrification by soil microorganisms when applied at rates as low as 0.5 (mu)g g('-1) soil could not be confirmed. No inhibition of denitrification was observed when these compounds were applied at the rate of 10 (mu)g g('-1) soil, and enhancement of denitrification was observed when they were applied at the rate of 50 or 100 (mu)g g('-1) soil;None of the 33 pesticides studied inhibited denitrification when applied at the rate of 10 (mu)g g('-1) soil, and only one of them (captan) inhibited denitrification when applied at the rate of 50 (mu)g g('-1) soil. The other pesticides studied either had no significant effect on denitrification, or enhanced denitrification, when applied at rates up to 100 (mu)g g('-1) soil. Reports that the herbicides atrazine, simazine and dalapon and the fungicides maneb, thiram and terrazole inhibit denitrification in soil could not be confirmed.