Different anabolic steroids can exercise different effects on the pituitary-gonadal axis in males. During a pilot study regarding the possible beneficial effect of the anabolic steroid nandrolondecanoate (ND) on bone metabolism in patients with rheumatoid arthritis additional endocrinological parameters were studies. A significant decrease was found in the serum levels of testosterone, androstenedione and FSH and the ratio of testosterone/oestradiol. There was a significant increase in the serum levels of oestrone. The levels of oestradiol, SHBG, LH and cortisol remained unchanged. An inhibitory effect of ND on testicular testosterone secretion is assumed. The decrease in androstenedione levels is explained by the diminished testosterone secretion. The rise in oestrone levels is explained by peripheral aromatizing of ND to oestrogens. The presented findings are in accordance with the hypothesis that sex steroids can act directly on the pituitary resulting in selective FSH and LH secretion. The possible role of the ratio testosterone/oestradiol in controlling gonadotrophin output is discussed.