The peripheral plasma concentrations of testosterone, luteinising hormone (LH) and follicle stimulating hormone (FSH) were determined in 46 patients (age 51-86 years) with cytologically confirmed prostatic carcinoma. Treatment consisted of subcapsular orchidectomy (15 cases) or estrogen medication (16 cases) or cyproterone acetate (15 cases). The determinations were made before and 2 weeks and 2 months after treatment was initiated. No correlation was found between the pretreatment levels of testosterone and gonadotrophins and the local extent of the primary tumor or the degree of malignancy. Nor was there any correlation between hormonal level, presence of metastases or patient age. Orchidectomy and estrogen medication both resulted in very low plasma testosterone levels, corresponding to 15% of the pretreatment values. This proportion was 28% in the group treated with cyproterone acetate. Orchidectomy was followed by significant increase in the levels of LH and FSH. Estrogen treatment resulted in suppression of the LH levels to 40% and of FSH to 14% of the pretreatment values. The corresponding decreases in response to cyproterone acetate were 65 and 35%. The results indicate that reduction in gonadotrophin secretion is the primary mechanism in the lowering of testosterone levels produced by treatment with estrogens or cyproterone acetate.