The pharmacokinetics and endocrine effects of the LHRH analogue buserelin [D-Ser(TBU)6-LHRH], released from biodegradable implants, were studied in 14 patients with stage C and D prostate cancer. Six patients received a subcutaneous implant of 3.3 mg buserelin monthly, and 8 patients received a subcutaneous implant of 6.6 mg buserelin every two months. Serum levels of buserelin decreased rapidly immediately after implantation. After 1-2 weeks a more gradual decline occurred, while in the two-monthly treated group a third phase of the elimination curve started after 5 weeks. Mean serum buserelin levels just before the next implantation in the two groups were not different. Urinary excretion of buserelin followed the same pattern. Serum LH levels in both groups became non-detectable 2 weeks after the first implant. This decrease of LH levels was accompanied by a suppression of serum testosterone to concentrations below 1 nmol/l (castration level). Side effects were not different from those observed with the intranasal application of buserelin. It is concluded that the subcutaneous application of buserelin is an easily administered form of treatment which has more profound and more reliable endocrine effects when compared with the intranasal administration of the drug. The greatest advantage of the new preparation is that the intervals between applications may be prolonged to at least 2 months.