In sea urchin eggs, Ca 2+ mobilization by nicotinic acid adenine dinucleotide phosphate (NAADP) potently self-inactivates but paradoxically induces long-term Ca 2+ oscillations. We investigated whether NAADP-induced Ca 2+ oscillations arise from the recruitment of other Ca 2+ release pathways. NAADP, inositol trisphosphate (IP 3) and cyclic ADP-ribose (cADPR) all mobilized Ca 2+ from internal stores but only NAADP consistently induced Ca 2+ oscillations. NAADP-induced Ca 2+ oscillations were partially inhibited by heparin or 8-amino-cADPR alone, but eliminated by the presence of both, indicating a requirement for both IP 3-and cADPR-dependent Ca 2+ release. Thapsigargin completely blocked IP 3 and cADPR responses as well as NAADP-induced Ca 2+ oscillations, but only reduced the NAADP-mediated Ca 2+ transient. Following NAADP-mediated release from this Ca 2+ pool, the amount of Ca 2+ in the Ca 2+-induced Ca 2+ release stores was increased. These results support a mechanism in which Ca 2+ oscillations are initiated by Ca 2+ release from NAADP-sensitive Ca 2+ stores (pool 1) and perpetuated through cycles of Ca 2+ uptake into and release from Ca 2+-induced Ca 2+ release stores (pool 2). These results provide the first direct evidence in support of a two-pool model for Ca 2+ oscillations.