In starfish Asterias amurensis, acrosome reaction is induced by three components of the jelly coat: ARIS, Co-ARIS and asterosap. ARIS plays a key role in triggering the acrosome reaction and asterosaps facilitate the ARIS function. Asterosap induced the increase of cGMP level in starfish sperm, as a receptor of asterosap is the guanylate cyclase. The resting level of cGMP was 0.4 pmol / 10(8) sperm and the cGMP level with 1 mM asterosap to increased by more than 100-fold in less than 400 msec and followed by a rapid decrease. The resting level of cAMP was 5.5 pmol / 10(8) sperm but cAMP levels did not change significantly with asterosap. Asterosap elicited rapid increase in calcium signal. These data show that the calcium entry mechanism in starfish sperm is much more sensitive to cGMP than to cAMP.