An in vitro experiment was undertaken to investigate the physiological role of estrogen on the neuromechanism, especially the release of eggs induced by serotonin, in the ovary of the scallop Patinopecten yessoensis. Estradiol-17β(E2) had no direct action on the release of eggs from the ovarian pieces, whereas preincubation with E2 accelerated the release of eggs induced by incuba- tion with 10-7M and 10-6M serotonin (5-HT), depending upon the E2 concentration. In ad-dition, the accelerating effect of E2 was inhibited with aspirin (cyclooxygenase inhibitor) and ac-tinomycin D (RNA synthesis inhibitor). Therefore, it was suggested that prostaglandin (PG), functioning as a neuromodulator on the 5-HT action in the spawning of the scallop, is one of the factors which cause the promoting effect of E2 on 5-HT action, together with the induction of a protein formation via a genomic action, and that E2 may enhance the sensitivity to external stimuli in egg release with the progressing maturity of the scallop.