Testosterone, estradiol, and dehydroepiandrosterone (T, E2, and DHA) were measured by RIA in plasma samples from three groups of sexually immature male and female (male, female) river lampreys: (I) intact, (II) gonadectomized, or (III) with pro- and mesoadenohypophysis removed (hypophysectomized). T levels were around 0.1 ng/ml in both I male and I female and increased significantly in all operated groups, especially in II male, to 1.2 ng/ml. E2 levels were between 0.7 and 1 ng/ml in I male and I female and were significantly elevated only in II male to 2.0 ng/ml. DHA levels showed significant decreases, from around 0.9 ng/ml in I male and I female, in both operated groups. It is suggested that T and E2 are secreted from a nongonadal source in both sexes and then converted to biologically active sex hormones in the gonads. Therefore T and E2 do not decrease, and may even accumulate, in the blood after gonadectomy. Since extirpation of the pro- and mesoadenohypophysis was not followed by a decrease in T and E2, their secretion may be stimulated by a hormone from another part of the pituitary such as the metaadenohypophysis.