Autoradiographs of the total alpha activity distribution in slices through manganese nodules can be used to determine growth rates and to investigate the mode of incorporation of alpha emitters into nodules. Total alpha vs. depth profiles, measured with high resolution by counting alpha tracks in cellulose nitrate detectors, are almost always exponential and apparently record nodule growth. Accumulation rates determined by 230Thex measurements are in approximate agreement with the alpha track measurements. Sediment accumulation rates may also be measured by this technique.