Autoradiographs of potato tubers (Solatium tuberosumL. cv. Mans Piper) grown in the presence of “Ca revealed high activity in the periderm, vascular ring (predominantly xyiem), phloem bundles and pith. It is calculated that only 40 per cent of tubeT Ca entered directly through the periderm and the possibility of import via the phloem is discussed. Chemical fractionation procedures showed that more than 90 per cent of tuber Ca could be considered to be in a physiologically active form and very little in the form of insoluble components such as calcium oxalate. Microautoradiography confirmed the infrequent occurrence of specialized calcium oxalate-containing cells. When tubers were sprouted in trays in the dark water-soluble forms of “Ca were preferentially transported to the sprout Greater depletion occurred from the outermost tissues of the tuber but there was no indication of preferential mobilization from tissues immediately adjacent to the sprouting region. Sprouts which developed sub-apical tip necrosis showed a positive gradient of “Ca from the tip to the base.