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Hormone release rates from biodegradable cylindrical implants consisting of a physical matrix of [14C]levonorgestrel and copolymers of [3H]lactic and glycolic acids have been monitored in rats. Two copolymers were evaluated: one consisted of 90 parts L-lactide/10 parts glycolide (90L/10G) containing 33 or 50% hormone by weight, and the other of 50 parts DL-lactide/50 parts L-lactide (50DL/50L) containing 50% hormone. For each system, 4-6 rods (0.8 x 16 mm) providing 19 mg of steroid per rat were subcutaneously implanted into the scapular regions of 5 rats, and 14C and 3H in faeces and urine were determined weekly for 90-724 days. An initial burst of hormone release, peaking at approximately 90 microgram day-1, occurred in the first two weeks. This was followed by an approximately 10-30 microgram day-1 for the 90L/10G system containing 33% hormone, a more uniform rate of approximately 25 microgram day-1 for the 50DL/50L system and the highest rate of approximately 40 microgram day-1 for the 90L/10G system containing 50% hormone. 3H and 14C in residual implants of 90L/10G with 33% hormone removed from animals dying of natural causes during the test were assayed. For this system 3H activity decreased by over 50% within 250 days, compared with < 25% loss in 14C activity. The amounts of 3H and 14C released were similar over much of the subsequent test period. At the end of the test both polymer and drug were essentially depleted. All animals with the 50DL/50L system died late in the test period. Between days 609 and 724 from 13.7-27.2% initial 14C and from 10.0-22.1% initial 3H was measured in recovered rods. Microscopic inspection of recovered rods showed a loss of core material and tissue encapsulation. There were no signs of local tissue irritation or systemic toxicity.