Understanding of pheromone function in teleost fish has been impeded by a lack of information on pheromone identities1. Our recent studies 2,3 on goldfish Carassius auratus, however, provide strong evidence that 17α,20β-dihydroxy-4-pregnen-3-one (17,20P), the proposed oocyte maturation-inducing steroid hormone in goldfish4 and other teleosts5, could be a potent female sex pheromone. Milt (sperm and seminal fluid) volume in goldfish is increased by exposure to 17,20P (and to a lesser extent by exposure to two precursors of 17,20P, progesterone and 17α-hydroxyprogesterone) but not to other steroids proposed as fish pheromones1,6,7. In addition, the goldfish olfactory epithelium is extremely sensitive to 17,20P (ref. 3), and the increase in milt volume normally induced by 17,20P exposure is abolished by sectioning the medial olfactory tracts, which previously have been implicated in the control of sex behaviour in male goldfish8. We report here that ovulating goldfish release 17,20P into the water and that a rapid (within 15 min) elevation in blood gonadotropin of males mediates the milt response to 17,20P exposure. We conclude that this pheromone system synchronizes milt production with ovulation.