The present work gives the results of extensive calorimetric measurements to determine the inhibitory effect of cadmium ions on enzymatic urea hydrolysis. Three milligrams of urease and 10 mmol urea (kg solution)(-1) were found to be the optimum reaction conditions in 0.05 M HEPES buffer (pH 6.6). A correlation between the initial rate v(0) and the cadmium concentration in the ppm range can be demonstrated by determination of the initial slope of the calorimetric measuring curves. By determination of the enzyme kinetic data, a contribution to detailed information on the inhibitor mechanism of cadmium can be made with these results.