For the purpose of examining the clinical exchange indication 96 removed Demand-pacemakers (type Medtronic 5842, 5843, 5942, 5943) had been sujected to a functional test by the manufacturers. After a service-life from 31-45 months 71% (i.e. 42 out of 59 units) and, in the group with a service-life from 31-35 months, 66% (i.e. 33 out of 51 units) of pacemaker units removed at that time were still working regularly. The mean energy loss exceeded 70% with a simultaneous mean drop of the output voltage down to 2.5 Volts. After the 25th month of the implantation date the number of battery failure increased spasmodically. Under the suspicion diagnosis of "battery exhaustion" a prophylactic "selective" exchange operation of the pace-maker units implied does not appear justifiable to us but in cases with stable AV-Block III. Providing reliable supervision of patients with sufficient self-rhythm, the exchange may be delayed up to about the 40th month after the implantation.