Quantitative analysis of the positive and negative feedback actions of testosterone (T) was carried out using intact rats, and orchidectomized rats implanted with T-filled Silastic capsules. Target organ weights and serum levels of LH and T were examined 7 days later, and response ratios of positive and negative actions were plotted against logarithm of serum T. From x-intercepts and slopes of regression lines, thresholds and responsiveness of reactions were calculated, respectively. Maintenance T levels necessary to maintain onset weights of the organ were also calculated from the regression lines. We compared 5 parameters at various ages, 1) thresholds for lowering serum LH, 2) mean T concentration of intact animals, 3) upper limit of serum T (mean + 2SD), 4) maintenance T levels, and 5) lower 95% limit of the thresholds for target organ response in orchidectomized animals. Though T can act between thresholds for target organ response and upper limit of serum T, the action range of T to induce organ growth over the onset organ weight (growth-inducing range) should be the area between the maintenance T level and the upper limit for serum T. At 3 weeks of age, the threshold for serum LH was very low, which makes the upper limit of serum T lower than maintenance T, allowing no growth-inducing range. From 5 weeks of age, the threshold for serum LH increased, and the serum T level with its upper limit also increased over the maintenance T level, allowing the presence of a growth-inducing range of T to make the growth of target organs possible.