Testosterone and androstenedione concentrations in the plasma and testis of male guinea-pigs were estimated by gas chromatography at intervals (36 stages) from birth to death. Four main periods of androgenic activity were recognized. The neonatal period, from birth to Day 16, is characterized by a precocious but transient peak in plasma testosterone concentration at Days 2 and 3. The pubertal period from Days 16 to 90 can be subdivided into a prepubertal period starting on Day 16 and marked by a sudden linear increase in plasma and testicular testosterone concentration together with an increase in testicular and seminal vesicle weight, and a postpubertal period, from Day 50 (the time of hormonal puberty) to Day 90, characterized by high and stable androgen levels while testicular and genital tract development continues. Adulthood spans the period between Months 3-6 and Month 24; plasma and testicular concentrations of testosterone and androstenedione are stable but lower than those observed during puberty. The period of senescence occurs between Months 24 and 28 and is marked by a fall in testosterone secretion and involution of the seminiferous tubules and accessory sex glands.