The Nuclear Enterprises PDR3 is a portable battery powered instrument covering a dose rate range from 0.5 mrad h sup - sub 1 to 50 rad h sup - sub 1 in air and a dose range from 0 to 4 rad in air. The detector is a Geiger Muller tube provided with a Nuclear Enterprises designed energy compensation sheath. The instrument is manufactured by Nuclear Enterprises Ltd. A full description is given. The instrument characteristics have been evaluated. The results are summarized and compared with the relevant requirements of the International Electrotechnical Commission for exposure rate monitors. Radiation, electrical and mechanical characteristics were examined, also environmental effects of temperature and humidity. (U.K.). Burgess,-P.H.