In the rat during the last 4 days of fetal life, the production of testosterone by the testis does not increase, whereas the plasma level of biological LH-like activity rises sharply. The present study was designed to test whether this phenomenon is attributed to the development of defects in the activity of the enzyme 17 alpha-hydroxylase and C17,20-lyase (called late steroidogenic lesions) during the end of fetal life. As this occurrence would lead to an age-related decrease in the ratio testosterone/progesterone, the testicular productions of these two steroids were evaluated on 18.5, 20.5, and 21.5 days postconception. Three different measurements were performed: (a) the in vivo testicular contents; (b) the in vitro secretions during a 120-min incubation in the presence or in the absence of 100 ng/ml ovine LH; (c) the testicular contents after these incubations. These measurements never revealed a decrease in the ratio testosterone/progesterone as a function of fetal age. It would appear that late steroidogenic lesions do not develop during the end of fetal life in the rat.