An inexpensive isodose plotter is described which uses major items already present in most radiology departments. A rectilinear scanner provides scanning motions and plotting table and two electrometers are used to measure the currents from silicon diodes used as reference adn probe detectors. A simple box of electronics, incorporating an integrated ratio circuit and 13 integrated circuit comparators, interfaces the electrometers to the scanner's printing mechanism. The device worked very satisfactorily and took between 30 and 60 minutes for each plot.