An experimental flowchart is shown. Young and middle-aged rats were given two pretests 2 days apart. One day later, they were anesthetized, and a blood sample was drawn to enable measurement of intact serum hormone levels. Then, rats were bilaterally castrated, and implanted with a Silastic capsule containing either testosterone or vehicle. Two weeks later, the post-tests were performed two days apart. One day after testing, rats were euthanized by perfusion; a blood sample collected at that time was used for measuring hormone levels following the castration and hormone treatment.