Twelve adult, female baboons were used to test the rate and duration of progesterone (P4) release from cholesterol (C) pellets implanted subcutaneously. Four different preparations were tested (3 baboons per group). Each group was treated with pellets containing a different sized distribution and P4-cholesterol ratio (P4:C). Group 1 = 0.25--0.5 mm, 59% P4:41% C; Group 2 = 0.25--0.5 mm, 65% P4:35% C; Group 3 = 0.5 -1.0 mm, 59% P4:41% C; and Group 4 = 0.5 -1.0 mm, 65% P4:35% C. All baboons received the identical quantity of P4 (500 mg). Following treatment, blood samples were obtained at selected intervals, and the levels of progesterone were determined by radioimmunoassay. Serum-progesterone clearance curves were parallel for each experimental group. The rate of release of progesterone calculated from the serum clearance curves for groups 1--4 were: 42; 55; 29; and 38 mg P4/week. The mean duration of release for experimental groups 1--4 were 12, 9, 17, and 13 weeks respectively. The continuous progesterone treatment inhibited normal baboon ovarian function during the period of progesterone release.
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