Analog methods of dose calculation have been proposed, with mechanical, optical, and electrical means of generating the dose distribution function (1). A commercial device now available and marketed under the trade name “Bivar”3 facilitates the use of analog computers for dose calculations. This device uses a resistive plate on which lines may be drawn in silver ink to represent the dose at intervals of 5 to 10 per cent. These lines are maintained at electrical potentials proportional to the dose. The resistive material produces a potential at intermediate points related to dose by the same proportionality factor. A metallic probe moved over the plate in the X–Y direction by a servomechanism picks up the potential to generate the dose as a function of two variables. As a first test of the system, the bivariant function generator and a curve plotter were hooked together through a servo loop to reproduce isodose curves for a single field of 2-Mev x-rays. It was found that isodose curves for any desired dose level are satisfactorily reproduced. It was also found that by multiplying the X–Y coordinates fed to the function generator by suitable constants empirically determined, isodose curves could be generated for square or rectangular fields of any size up to 20 × 20 cm., with the use of only three function plates. These plates are drawn for 5 × 5-cm., 10 × 10-cm., and 15 × 15-cm. fields. Furthermore, by adding a fractional part of the width variable to the depth variable in the function generator, fields could be generated for oblique incidence up to 45°. It may be possible to simulate the patterns produced by wedge filters by use of the same three function plates. Accuracy of reproduction was of the order of 2 per cent of the surface dose or 2 mm., depending on position in the field. Using electronic analog technics, a computer has been set up to sum the dose over two fields incident on a patient. The fields generated by the two function generators are adjusted in patient space by analog computer technics. Either may be rotated to any position over a full 360° or translated in either the X or Y direction. The position of the maximum dose is then located and the value of this dose in terms of the surface dose of one of the fields is read. With the origin transferred to this point, the servo loop is closed and isodose lines at the chosen dosage levels are traced by the plotter. The method can be applied to three or more fields by expanding the system with the same number of function generators.