P. sterna oocytes cannot be fertilized in vitro unless they are obtained by inducing spawning. The present work was undertaken to develop a simple method to induce germinal vesicle breakdown that would enable the oocytes to be fertilized without prior spawning. We found that Pteria sterna oocytes removed from the ovaries are induced to undergo germinal vesicle breakdown (GVBD) by 10 mM NH4Cl in seawater, and proceed to metaphase I of meiosis, at which stage they can be fertilized and will develop at least as far as "D" larvae. On the other hand, ovarian oocytes exposed to 2 × 10–5 M serotonin in seawater remained in prophase and did not undergo GVBD. Ammonium chloride treatment provides a simple, inexpensive procedure that can be useful in studies of fertilization and development, and for attempts to repopulate this exploited species.