Contrary to prediction, a large intraspecific variation can be observed in reproductive timing of free-spawning marine organisms. The relative contributions of heritable genetic differences and phenotypic plasticity to this variation have remained unstudied. To do so would require experimental extension of the natural spawning season in cases where the windows of gamete release do not overlap. We tested the use of the selective serotonin re-uptake inhibitor fluoxetine (the active ingredient of Prozac) to initiate spawning in the marine bivalve Macoma balthica. In 5 experiments we attempted to induce spawning, applying 1 ppm fluoxetine versus a control group to M. balthica which were kept in the lab for up to more than a month after the natural spawning season. Both the fluoxetine and the control group received a preceding temperature shock, which is indispensable for the onset of spawning. We conclude that fluoxetine effectively induces spawning in M. balthica and helps obtain fertile gametes after the natural spawning season. We discuss the usefulness of fluoxetine as a tool for studying the origin of intraspecific variation in reproductive timing.