A discussion of the merits of the guarded-field type of ionization chamber as compared with the conventional type of thimble chamber is illustrated, with outlines of the electrostatic field distributions. Details of the mode of construction of a practical and robust guarded-field chamber are given. With the exposure dose rates (of the order of 100 r per minute) for which the chamber was designed it becomes feasible and advantageous to use the chamber in conjunction with a high-sensitivity moving-coil galvanometer instead of a Townsend Balance. A suitable galvanometer is described, together with a calibrating network which enables the ionization current to be determined by a null-type matching procedure.The results of Boag's theory of general recombination in a parallel plate type of chamber are summarized and it is shown that, as predicted by the theory, a plot of the current, j, against the square of the ratio of the current to the polarizing voltage (j/V)², yields a straight line which can be extrapolated to obtain the saturation current. The result of an absolute measurement of the exposure dose rate of The London Hospital cobalt unit is compared with the output as determined by a Victrometer dosemeter calibrated at the National Physical Laboratory.