The body and testis weights increase linearly with time during growth of the immature rat. Between 20 and 30 days of age, there is an increase in lactate dehydrogenase (LDH) activity in the interstitial and tubular tissue of the testis and glucose-6-phosphate dehydrogenase (G-6-PDH) in the interstitial cells. LDH-X isoenzymes appear between 20 and 30 days of age and are associated with the seminiferous tubular epithelium. There is a linear increase in seminiferous tubule diameter with age until 60 days, after which time there is no further tubular growth.