Abstract: An abnormally high pulse rate is an indicator of personal stress and can therefore be used to test persons for a guilty frame of mind which may, for instance be caused by an attempted theft by an employee from the premises where he works. An instantaneously measured pulse rate is compared with an average pulse rate computed from a past history of pulse rate measurements conducted in respect of the particular person under test. A discernable signal is produced if the instantaneous pulse rate exceeds the average by more than a predetermined amount. In cases where there is no average pulse rate, for instance where the person under test is a first time visitor to the premises, the instantaneous pulse rate is compared with a predetermined pulse rate.