The subject of this publication is calorimetric investigations on the inhibitor effect of heavy metal ions on the enzyme urease. The obtained results allow quantification of the inhibitors (Cd2+-, As3+-, Zn2+-, Pb2+-ions) via the initial reaction rate of enzymatically catalysed urea hydrolysis. The interpretation potential of the calorimetric measurements is underlined by the determination of the inhibiting mechanisms for the example of Cd2+- and As3+-ions, the findings on the time regime of the inhibition process, and the detection of heavy metal ions in the ppm range. The use of several different buffer substance revealed the influence of the latter on the intensity of heavy metal inhibition. This opens the path to both the selective analysis of heavy metals via pattern recognition and to the improvement of detection sensitivity.