Levels of sulphated 17α20β‐dihydroxy‐4‐pregnen‐3‐one (17,20β‐P; the oocyte maturation inducing steroid) in blood plasmas of sexually mature male and female rainbow troutOncorhynchus mykiss, were very low in comparison to those of the free steroid. However, relatively large amounts were found in urine of both sexes.Catheters were inserted into the urinary bladders of unovulated and ovulated females and of ripe‐running males, and the fish then placed in spawning channels. Three‐hourly urine samples were collected between 09.00 and 18.00 hours and then a 15‐h sample between 18.00 and 09.00 hours the next morning. Measurements were made of 17,20β‐P‐sulphale, testosterone glucuronide (T‐G) and 17β‐oestradiol glucuronide (E2‐G). In females, the highest rates of excretion of E2‐G, T‐G and 17,20β‐P‐sulphate were found in unovulated, ovulating and ovulated females, respectively. The rates of excretion of 17,20β‐P‐sulphate, T‐G and E2‐G in ovulated females were unaffected by the presence of a male. id males, however, there was a sharp increase in the rate of excretion of 17,20β‐P‐suiphate and T‐G in fish which were paired with an ovutated (nesting) female. A similar increase was