The effects of calcium-free sea water (CaFSW) on the starfish ovarian structure and on ovarian responsiveness to radial nerve factor (RNF) were studied following i n vitro incubation. Incubation of ovaries in CaFSW resulted in a progressive elongation of the entire ovary and the dispersion of follicular cells from around the oocytes contained within the ovary. The changes in ovarian length but not the follicular cell detachment were reversible when ovaries were placed in sea water containing calcium. Addition of Mg++ or Na+ to CaFSW did not prevent ovarian elongation or follicular cell dispersal. Strontium ions prevented and reversed ovarian elongation when added to CaFSW; however, follicular cell dispersion was not in- hibited. The oocyte shedding, normally induced by RNF, was prevented by maintaining the ovary in CaFSW. The degree of shedding inhibition was dependent upon the duration of washing in CaFSW. RNF did not prevent or reverse the ovarian elongation induced by CaFSW. The inhibitory action of CaFSW on RNF-induced oocyte shedding could be reversed by the addition of Ca++ or Sr++ but not by the introduction of Mg++ or Na+. The degree of reversibility was dependent upon the concentration of Ca++ ions. The data strongly indicate that contractile tissue in the ovarian wall is important in mediating oocyte shedding in response to RNF.