The hydrolysis of urea in phosphate buffer (pH 7.0) at 25 degrees C in the presence of sodium fluoride as an urease inhibitor was studied. It was found that fluoride ion is a competitive slow-binding inhibitor of the enzyme and the constants of inhibition were measured. The progress of the reaction was observed by means of a classical analytical method (increase of NH3 concentration) and a calorimetric method using an isoperibol batch apparatus with an accuracy of 0.001 K.