Variations in the metabolic clearance rate and the production rate of testosterone during puberty in the male guinea-pig were established by single injection and constant infusion methods. Similar results were obtained; MCR (1/24h) increases from Day 20 to Day 70 and then stabilizes between Days 70 and 90 to levels comparable to those of a 6-month-old adult. MCR as 1/24 h/100 g decreases steadily between Days 20 and 60. The evolution of the testosterone production rate (PR: ng/24 h) is characterized by a marked rise between Days 20 and 70, and then a decrease up to Day 90, continuing until 6 months. PR as ng/24 h/100 g increases between Days 20 and 40, remains stable until Day 70, then decreases. Testosterone half-life is stable throughout the pubertal period. The rate of conversion of testosterone to androstenedione remains stable except for a rise between Days 50 and 60. We conclude that the increase in testosterone concentrations during puberty is due to a rise in the production rate and not to a decrease of clearance rate.