A direct-reading probe type dosage-rate meter suitable for radium or X-ray measurements inside the human body is described. The instrument utilises an “air-walled” chamber of 0·07 c.c. volume together with a miniature thermionic electrometer system, the whole comprising a probe which may be shaped to specific requirement and be readily capable of insertion into the major body cavities. The electrometer output is conveyed by a flexible multicored cable of any desired length to the amplifying and indicating system which may be housed at any convenient distance. The most sensitive range of the instrument gives full scale deflection for five r/min., but the sensitivity is easily adjustable from this figure up to several hundred r/min. if required. Standard components are used throughout, the electrometer system being comprised of miniature radio valves. Some clinical uses to which the instrument has already been put in this hospital are described.