Ovarian oocytes of the mouse, Spisula and Chaetopterus are arrested in the dictyate stage of meiotic prophase. Upon isolation, mouse and Chaetopterus oocytes undergo spontaneous maturation manifested by germinal vesicle breakdown (GVBD) while Spisula oocytes retain their germinal vesicles. The present report describes a substance isolated from Spisula with meiotic arresting activity. The substance was purified from Spisula tissues by 70% ethanol extraction, chromatography on a Dowex l-x8 column, and reversed phase HPLC. GVBD in Spisula oocytes can be induced by insemination (5 x 104 sperm/ml) or treatment with serotonin or KCl. A crude ethanolic extract of Spisula tissues blocked oocyte maturation induced with serotonin. Upon washing in natural seawater (NSW) the oocytes proceeded to undergo maturation. Forskolin and 3-isobutyl-1-methylxanthine (IBMX) blocked GVBD induced with sperm or serotonin. Dibutyryl cyclic adenosine 3',5'-monophosphate (dbcAMP) and dibutyryl cyclic guanosine 3',5'-monophosphate (dbcGMP) blocked GVBD induced by sperm, while higher concentrations of these nucleotides were required to block serotonin-induced GVBD. However, none of the compounds tested including the Spisula extract influenced KCl-induced GVBD. Isolated Chaetopterus oocytes suspended in artificial seawater (ASW) retained their GV and underwent spontaneous GVBD when placed in NSW. Spisula extract, dbcAMP and dbcGMP inhibited maturation of Chaetopterus oocytes suspended in NSW. dbcGMP, however, was a more potent inhibitor than dbcAMP, suggesting that cGMP may be the factor that maintains meiotic arrest in Chaetopterus oocytes. Spontaneous maturation of mouse oocytes was blocked by HPLC-purified Spisula factor at a concentration of 5 µg/ml in combination with 50 µM dbcAMP. We conclude that a substance found in Spisula tissues sustains meiotic arrest in mouse, Spisula, and Chaetopterus oocytes.