Although blood-borne prostaglandin F2a (PGF2a) is known to synchronize ovulation with sex pheromone release and female sexual receptivity in many fishes, how it enables female receptivity is not understood. This study examined the possibility that one way PGF2a acts is by altering how female fish perceive pheromonal odors. Specifically, we addressed the possibility that mature male goldfish release a distinctive androgen-based odor which only goldfish with elevated PGF2a discern as a sex pheromone. Testing behavioral responses of both PGF2a-injected and saline-injected females in a maze, we discovered that while the latter were attracted to the odor of both immature and mature goldfish of both sexes, PGF2a-injected goldfish were only attracted to the odor mature males. Next, we discovered that androstenedione (AD), a C19 steroid known to be detected by the goldfish olfactory system, is part of this male odor. Not only was AD moderately attractive to PGF2a-injected females, but it became highly attractive when added to odor of immature goldfish odor that had previously been unattractive showing it is part of a mixture. Species-specificity was later demonstrated in behavioral tests which added ng quantities of AD to the odor of several other fishes but found only the odor of goldfish to be fully active. Both PGF2aand AD are common fish hormones and it seems reasonable to ask whether they might also play dual overlapping roles as pheromones and hormones mediating reproductive synchrony in other fishes. (Supported by the National Science Foundation).