Starfish (Asterias forbesi) oocytes encased within their follicle cells mature spontaneously during a portion of the normal reproductive period when released from the ovary into seawater. A previous report has shown that oocytes isolated in acidic seawater do not mature spontaneously but retain the capacity to do so when returned to normal seawater. The object of this study was to determine the mechanism by which acidic pH reversibly blocks spontaneous oocyte maturation in isolated follicles. Incidence of spontaneous oocyte maturation in follicles isolated in acidic seawater decreased as pH decreased from 7 to 4. Oocytes in which spontaneous maturation was inhibited (ASW at pH 4.7 TO 5.4) underwent germinal vesicle breakdown with the addition of 1-methyladenine. Oocytes isolated in acidic seawater (pH 4 or 5) with intact follicle cells matured spontaneously when transferred immediately to normal seawater pH 8); after four hours, 60-65% of the follicles incubated in seawater at pH 5 matured spontaneously when returned to normal seawater as compared to less than 10% of the follicles maintained at pH 4. Inhibition of spontaneous maturation was not reversible in the absence of the follicle cells. Oocytes isolated in acidic seawater with their follicle cells did not spontaneously mature when transferred to calcium-free seawater at pH 8. The results obtained support the hypothesis that acidic seawater reversibly inhibits spontaneous oocyte maturation by interfering with the release of meiosis-inducing substance from the follicle cells.