The postnatal development of body and testis weight and the size of the testicular cell populations were studied in CBA mice up to day 52 post partum . The body weight increased from 1.3 g at day 1 to 22.5 g at day 52. Over the same interval the testis weight showed a faster increase from about 1 mg to almost 60 mg. Spermatogenesis was found to be complete by day 35. The numbers of A spermatogonia, Sertoli cells, Leydig cells, mesenchymal cells, macrophages, myoid cells, lymphatic endothelial cells, endothelial cells and perivascular cells per testis were studied from day 3 to day 50, using the disector method. The number of A spermatogonia increased from 0.2 × 10 ⁵ at day 3 to 6.5 × 10 ⁵ at day 21 and remained more or less constant thereafter. The Sertoli cell population increased during the first three weeks after birth to reach the adult level of approximately 18 × 10 ⁵ cells per testis. In the interstitium the Leydig cells showed a sharp increase between days 11 and 31 followed by a small decrease to ultimately 9 × 10 ⁵ cells per testis. The Leydig cells formed 8% of the total number of interstitial cells per testis at day 11, increasing to 30% at day 50. The number of mesenchymal cells did not change until day 36, decreasing thereafter from about 2.5 × 10 ⁵ to 1 × 10 ⁵ cells per testis at day 50. However, the percentage of the total number of interstitial cells that were mesenchymal cells decreased from 59% to 4%. The number of macrophages per testis increased very slowly from almost zero at day 3 to about 1 × 10 ⁵ at day 28. Except for a sharp peak of 2.5 × 10 ⁵ at day 31, this level was maintained. The populations of myoid cells, lymphatic endothelial cells and endothelial cells showed major increases during the third, fourth and fifth week after birth to 6.5 × 10 ⁵ , 3.7 × 10 ⁵ and 4.5 × 10 ⁵ cells per testis, respectively. The total number of perivascular cells was found to be approximately 2 × 10 ⁵ and did not change significantly. These results show that the major cell populations in the mouse testis have reached their adult size by the end of the fifth week after birth. The changes in numbers per testis of the various cell types during development were found to be closely related to their proliferative activity.