An integral dynamic reactor has been used for the continuous measurement of the instantaneous polymerization rate. With ethylene as monomer, the rate goes through a maximum after a few minutes, then levels off, and finally decreases owing to the blocking of the surface by the polymer. The catalytic activity may be calculated safely from the maximum instantaneous rate, after correction for the monomer consumption along the column of catalyst. As examples of application, it is shown that the catalytic activity goes through a maximum with either the chromium content at a fixed temperature of polymerization or with the temperature at a fixed chromium content.