A new method for stereometric measurement of surface area in microscopy is described, based on the mixed-image principle. The measurement was accomplished by observing the area on which was superimposed the image of a miniature electric lamp attached to an ordinary planimeter. Mixed-image planimetric measurements were more easily made and required much less time than area calculation by an integrating eyepiece, by weight or by ordinary planimetric measurements of photomicrographs or tracings of the area. Mixed-image planimetric measurements of a finite area varied less than 1 per cent. Such precision was not obtained with the other methods. The new method was devised in vivo stereometric measurements of bone tissue remodelling in rabbit ear chambers, but should find a much wider application in quantitative microscopy of both finite and extensive objects.