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Plasma testosterone levels in adult and neonatal female rats bearing testosterone propionate-filled silicone elastomer capsules for varying periods of time.

Testosterone propionate (TP) was administered, by means of subcutaneous implanted silicone elastomer capsules, into adult and neonatal (aged 3 days) female rats. In the adult rats a dose-dependent increase in plasma testosterone was measured for capsules of three different sizes (5, 10 and 20 mm crystal length). Testosterone levels reached a peak 4-8 h after insertion (5 mm, 24.6 +/- 1.4 (S.E.M.) nmol/l; 10 mm, 34.0 +/- 3.8; 20 mm, 44.4 +/- 3.1) and returned to control levels within 4 h after removal: the calculated half-life of testosterone was 1 h for all sizes of capsule. In neonates, a capsule of 2.5 mm crystal length was removed after 4 h subcutaneous implantation (at day 3 of age) and produced peak testosterone levels of 126.2 +/- 11.8 nmol/l: the calculated half-life was 8.6 h which compared with a half-life of 48 h after a subcutaneous injection of 312 mumol TP (in 0.05 ml arachis oil) which produced peak levels of testosterone in 4-8 h of 84.6 +/- 11.8 nmol/l. Chronic implants of TP-filled capsules (2.5 mm crystal length) at 3 days of age and left in situ for 15 weeks gave a half-life of 69 h. Removable silicone elastomer capsules were found to be a versatile vehicle for the administration of TP to rats of all ages where precise hormone treatment for a known period or prolonged administration is required. The duration and magnitude of plasma hormone levels should be established by assay in an in-vivo situation.