Minced and teased-tubular preparation obtained from rat testicular tissue (Holtzman strain) from animals of various ages were incubated with either pregnenolone-7α-3H and progesterone-4-14C or 5-HT-2-14C. MAO activity (EC 1.4.3.4) was estimated by the amount of 5-HIAA-2-14C that was produced from the 5-HT-2-14C. Androgen synthesis was estimated by the amount of androstenedione and testosterone that was recovered from the incubations. MAO activity, expressed on either a per mg of tissue basis or per 100 g bw basis, was high in testicular preparations from neonatal animals, was reduced in preparations from slightly older animals, was elevated during sexual development, and decreased again with advanced age (after 365 days). Androgen synthesis on either a per animal basis or a per incubation basis was low for 30-day-old rats. It was higher for 90-day-old rats, but was diminished for 410-day-old rats. Both pregnenolone and progesterone were converted to 17α-hydroxyprogesterone, androstenedione and testosterone. Radioactively labeled serotonin accumulated in small amounts in mature rat testes 120 min after sublingual injection. The changes in MAO activity were positively correlated with changes in androgen synthesis and testicular development with respect to aging. Senescence occurred in the testis with respect to androgen synthesis and MAO activity after 105 days of age and preceded a noticeable weight change that was evident in this investigation at about 410 days of age. (Endocrinology90: 1610, 1972)