The changes in plasma concentrations of eight steroids (testosterone, testosterone glucuronide, estradiol, estradiol glucuronide, 17-hydroxyprogesterone, 17,20 beta-dihydroxy-4-pregnen-3-one (17,20 beta P), deoxycorticosterone, and cortisol) have been followed in three individual carp, Cyprinus carpio, during ovulation induced by carp pituitary extract. Deoxycorticosterone and estradiol glucuronide were not detectable and small amounts of 17,20 beta P were found only in one fish. A priming injection of pituitary extract, administered 24 hr before ovulation, stimulated an increase in testosterone 3-6 hr later, followed by an increase in estradiol. The second injection of pituitary extract given 12 hr after the priming dose, resulted in a second peak of testosterone which was accompanied by a peak of testosterone glucuronide, a conjugate which was usually formed in only small amounts following the priming dose. A sharp peak of 17-hydroxyprogesterone followed the second stimulation and it is suggested that this, rather than 17,20 beta P, may be the natural inducer of maturation in carp. Although cortisol levels showed large variations, they appeared to be stimulated by the priming dose of pituitary extract. Levels of all steroids, except for cortisol, fell rapidly after ovulation.