It is now well established that female goldfish release unmodified and metabolized sex hormones to the water and that some of these compounds function as potent sex pheromones detected by the male's olfactory sense. In goldfish, both olfactory pheromonal receptors and their corresponding hormonal receptors appear to be transmembrane-domain receptors coupled with G proteins. Recent studies of other teleost fish indicate that fish commonly use `hormonal-pheromones.' Taken together, these data suggest that fish pheromone systems may have evolved as a consequence of a chance expression of hormone receptor molecules on olfactory receptor cells. Isolation and identification of olfactory and hormonal receptors may be the next step in resolving this question.