It is well known that conventional depth-dose tables represent the distribution of exposure dose along the central axis of an x-ray beam in a homogeneous, unit density phantom of effectively infinite dimensions. Clinical material is rarely represented by this model and therefore the data thus obtained will be subject to corrections in many clinical situations. Corrections may be obtained by two different paths. One approach is to investigate the effect of limitations of dimension on depth-dose data by reducing the size of the phantom. Data obtained by this method have been described in the literature (1, 2). Such corrections are relatively easy to apply, but further corrections for inhomogeneities, such as bone, presuppose an accurate knowledge of the thickness, extent, degree of calcification, etc., of the interfering structure (3). Such information is rarely available. Another approach has been to measure tumor exposure dose in a number of standardized situations in vivo or in cadavers (4, 5, 6), comparing the data thus obtained with conventional tables. Factors may then be obtained which permit tumor doses to be computed from standard depth-dose tables for reproductions of these standardized situations. Earlier work with this approach showed a small discrepancy between measurements in vivo and in cadavers. Also, much of the work was limited, in the deep therapy range, to 200 kvp and, in the supervoltage range, to 700 kvp. It was desired to extend this type of approach, encompassing corrections for limitations in size and inhomogeneities in one factor, by in vivo measurements in the 250-kvp range in deep therapy and 2 Mev in the supervoltage range. For this purpose an ionization-chamber dose-rate meter instrumentation was chosen, in preference to condenser chambers, as it would permit gross errors of alignment to be promptly observed and adjusted, and measurements so made would not interfere with the normal treatment pattern of the patient. The dose-rate meter circuit was patterned after that of Fedoruk (7). This circuit was chosen because of its known characteristics of linearity, stability, portability, ease of maintenance, and lack of drift with short warm-up time. The ionization chamber was designed with the following requirements in mind: 1. The chamber and its connecting flexible cable should be of small enough dimensions to permit insertion into sites such as the esophagus, nasopharynx, male bladder, etc., in vivo, under local anesthesia at the most, and with minimal trauma. 2. The flexible cable should not give rise to spurious signals. 3. The chamber tip should be waterproof and suitable for cold sterilization. 4. The chamber should be mechanically robust and unlikely to shed components in use. 5. The chamber volume should not be subject to changes due to flexion of the cable.