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 dissector method. The number of A spermatogonia increased from 0.2 x 10(5) at day 3 to 6.5 x 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 x 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 x 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 x 10(5) to 1 x 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%.(ABSTRACT TRUNCATED AT 250 WORDS)