The frequency of a laser can be controlled by mounting one of the resonator mirrors on a piezoelectric transducer and applying a control voltage to the transducer. The proper design of an automatic frequency control system requires more information about the transducer's dynamic behavior than is available from manufacturer's data. This dynamic behavior can be measured by applying an alternating voltage to such a transducer in an operating laser and using an optical heterodyne FM receiver to measure the frequency modulation produced. Comparison of the control voltage applied to the transducer with the output of the FM demodulator can yield useful information about the dynamic response of the transducer. Instrumentation used to make such measurements is described and some typical data are presented.