The effectiveness of Stay-N 2000 or reformulated nitrapyrin [2-chloro-6-(tricholoromethyl) pyridine was investigated in two Iowa soils representative of Clarion and Okoboji soils that differed in organic carbon, pH, and texture. A nonlinear regression was used to estimate kinetic parameters. The maximum nitrification rate (Kmax) and the duration of lag period (t') were derived from an equation to characterize the nitrification process in both soils. Stay-N 2000 appeared to be a better inhibitor than nitrapyrin to extend the t' and as effective as nitrapyrin in reducing the Kmax. Stay-N 2000 reduced the Kmax an appreciable amount in the Okoboji soil at the rate of 12 µg a.i. g-1 soil or three times of the recommended rate. Nitrification rates were affected by the rates of N applied to both soils, the higher the N rates, the higher the Kmax and the more the NO3--N accumulation.