A low cost spot densitometer system is described. This system is useful for quantitative autoradiography of local cerebral glucose utilization, blood flow, receptor binding and other applications requiring densitometry on films. The densitometer can be used alone or interfaced to a microcomputer. The densitometer consists of a photographic enlarger, a digital multimeter, and the densitometer electronics. We have described how to construct, test and use the densitometer and how to interface the densitometer to a microcomputer. The advantages of this system are: (1) the ability to enlarge the image for accurate measurements from 'small' areas; (2) a completely unobscured image during measurement; (3) low cost and (4) ease of use.