FIELD: measurement of activities of liquids containing radioactive substances. SUBSTANCE: measured liquid flows through tube and past end 1 of probe 2 mounted for turn. Measurement end of probe 2 with slit 15 open into tube remains dipped in liquid which level is kept preferably constant thanks to presence of overflow partition 19. With turn of probe 2 measurement surface opens into another duct 23 over which flushing fluid runs, hits tube and washes it through. EFFECT: capability for fast and reliable flushing of probe without its disassembly. 5 cl, 8 dwg