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The rate of hydrolysis of urea in soil over the wide range of concentrations, up to 10 moles N per dm3 soil solution, found in fertilizer practice, was examined in Begbroke sandy loam adjusted to different p H values.On rewetting air-dry soil, urease activity increased rapidly, reached a maximum within the first 24 h and then decreased slowly to level off after about 4 days. Pretreatment of the soil with urea or ammonium had n o effect on the urease activity. Urease activity increased with substrate concentration, reached an optimum value and then decreased with rising urea concentration. The results could be explained by substrate inhibition at higher urea concentrations, and the data are well described by a modified Michaelis-Menten equation involving three parameters, V,,,, K , and Ki where Ki is an inhibition constant. K , decreased linearily with rise in pH whereas Ki increased slightly between pH 4.9 and 7.0 and steeply between 7.0 and 8.4. V,,, increased with rise in pH, reached a maximum value a t pH 6.0 and then declined a t higher pHs.There was a further reaction, reaching a maximum rate at a urea concentration of about 0.2 molar N in the soil solution, that followed Michaelis-Menten kinetics. K, for this high affinity reaction increased up to pH 7.2 and then decreased a t higher pH values; V,,, increased up to pH 6.8 and then decreased. The contribution of the high affinity reaction was small except at low concentrations of urea.