Testosterone-filled polydimethylsiloxane (PDS) capsules (0·245 mm wall thickness) were placed subcutaneously in normal adult male rabbits to assess the relationship between the surface area of the implant and the amount of testosterone released. Testosterone release was quantified in terms of plasma testosterone concentrations and other biological endpoints such as libido, accessory sex organ weight, seminal fructose and citric acid levels and spermatogenic activity. Subcutaneous placement of PDS capsules ranging from 100 to 800 mm2 in surface area had no measurable (P>0·25) effect on (1) plasma testosterone concentrations, (2) libido, (3) accessory sex gland weight, and (4) the concentrations of fructose and citric acid in seminal fluid. The total daily sperm production and the number of step-6 spermatids decreased linearly with each successive increase in capsule surface area. It was concluded that testosterone-filled PDS capsules (800 mm2) induced azoospermia in rabbits without any significant (P>0·25) alteration in either plasma testosterone or the weight and secretory activity of the accessory sex glands.
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