A laboratory study at different metal loadings was done in two Iowa soils varying markedly in organic C (OC), pH, and texture to measure heavy metal effects on nitrification. Two soils were treated with Stay-N (reformulated nitrapyrin). A 10-g sample of each of the soils was treated with a solution containing 2 mg N, 4 µg a.i. g-1 soil of Stay-N, and 50 µmol of the following heavy metals Cu(II), Zn(II), Cd(II) as sulfates, and Pb(II) as acetate. All soils were incubated at 20 ± 1 C for 0, 7, 14, 30, 45, and 60d. A first-order equation was used to calculate the maximum nitrification rate (Kmax), duration of lag period (t'), period of maximum nitrification (Ät), and duration of lag plus maximum nitrification period (ts). All metals used in this study reduced Kmax and extended the t' in both soils. In the Clarion soil (24.0 g kg-1 OC), Kmax was 12 ± 1 mg kg-1 d-1 without the addition of Stay-N but reduced to 4; 0.25; 0.86; and 0.27 mg kg-1 d-1 when Stay-N and Cu, Zn, Pb or Cd were added, respectively. Kmax in the Okoboji soil was 22 ± 2 mg kg-1 d-1 without the addition of Stay-N but reduced to 6, 3, 4, and 2 mg kg-1 d-1 when Stay-N and Cu, Zn, Pb or Cd were added, respectively. The t' of Cu, Zn, Pb or Cd and Stay N varied from 8 to 25 d in the Clarion soil and from 5 to 25 d in Okoboji soil. The influence of metals added with Stay-N on the Kmax and the t' varied among soils indicating that other factors inherent in the soils also affect nitrification.