Levels of prostaglandins E(2) and F(2alpha) were measured in male and female gonadal portions of the functional hermaphroditic scallop Argopecten purpuratus, at the time of spawning. For these measurements, mature scallops were stimulated to initiate their chain of spawning events, and prostaglandins (PGs) were analyzed in samples before stimulation, when animals appeared stimulated to release sperm, during the release of sperm, when animals appeared stimulated to release oocytes, when most of the oocytes had been released, and 24 hr after the end of spawning. The experiment was run twice, once in winter and once in spring. An additional in vitro experiment was carried out to analyze activity of the monoamines dopamine and serotonin on the levels of these prostaglandins. For this, minced tissue of the female portion of the gonad was incubated for different time periods with each of the amines at 10(-5) M. PGs were determined in the tissue samples by radioimmunoanalysis. The results showed that the amounts of the PGs increased significantly in both parts of the gonad during the global process of spawning and decreased when it had been completed. Incubation of minced gonad with the amines affected the time course of PGs variations, i.e., the effect of these compounds depended on the incubation time used. The results support a model for the regulation of spawning which assumes the occurrence of a stimulus which is detected by receptors, processed by nerve cells, and sent to the gonad where intermediation by amines (changing the time course of the process) induces liberation of gametes, in some way modulated by prostaglandins.