Administration of the superagonist analog of GnRH, D-Leu6-GnRH proethylamide, profoundly reduced plasma LH, FSH, testosterone, and dihydrotestosterone levels when given for 6-11 weeks to adult men with prostatic carcinoma. Since patients with prostatic carcinoma can be expected to receive this analog for as long as 3-4 yr, we questioned whether the same degree of reduction could be maintained during chronic administration. In 22 men who had received D-Leu6-GnRH proethylamide for at least 1 yr, LH and testosterone remained at the initial low levels. Plasma dihydrotestosterone concentrations, on the other hand, gradually fell further with long term administration. FSH levels reached a nadir of 5.7 +/- 0.94 (+/- SEM) mIU/ml at 10-11 weeks. Unexpectedly, the plasma levels of this gonadotropin then gradually increased, and between 25 and 97 weeks were approximately 10-15 mIU/ml. This pattern occurred identically in patients receiving either 1 or 10 mg D-Leu6-GnRH proethylamide daily. These data indicate persistent suppression of LH and androgen levels during prolonged therapy and suggest that D-Leu6-GnRH-induced "medical castration" can be maintained with chronic administration.