Male goldfish, Carassius auratus, exposed to the steroidal pre-ovulatory pheromone 17α,20β-dihydroxy-4-pregnen-3-one (17,20βP) exhibited behaviour characteristic of sexual arousal within 30 min of exposure. These fish remained sexually aroused for 10 h when exposed to 17,20βP during scotophase: This period of time naturally separates the start of 17,20βP release and ovulation/spawning. Behavioural changes associated with overnight exposure to 17,20βP resulted in greatly improved spawning success in competitive spawning tests conducted the next morning. These males also produced more spermatozoa with greater motility. A daily pattern of pheromonal responsiveness was indicated. The function of the 17,20βP priming pheromone appears to be to enhance male spawning success by both physiological and behavioural mechanisms.