An external-storage telemetrical device for continuous detection and registration of radionuclide tracers in humans is described. The weight of the system is 600 g. A pocket-sized ECG cassette tape recorder with 24 h continuous registration constitutes the storage unit. Two end-window GM [Geiger-Mueller] detectors are used as surface detectors. Both .gamma.- and .beta.-emitters are detected by this equipment. The linear range is 100 counts/s. Renal excretion of [125I]iodide during 30 h in a normal person is shown. The disappearance rates are 0.1 h-1 during the day and 0.05 h-1 during the night. The coefficient of variation of the individual slopes has been calculated to < 5%. A comparison with the analog radiotelemetrical equipment is shown. The storage telemetrical equipment can be applied outside the hospital, when the patient is ambulatory.