The objective of this study was to describe the maturational changes observed in the seminiferous tubules of the monkey Cebus apella, a New World primate species, from birth to the end of puberty. Nineteen animals were subdivided into four groups: neonatal (1-40 days), infantile (4 months to 1 yr), early pubertal (1 yr, 8 months to 2 yr, 9 months), and late pubertal (4-8 yr). Volumetric determinations of different testicular components were made, tubule diameter and length were calculated, and spermatogenic cells, Sertoli cells, and androgen-binding protein secretion were quantified. Testicular and seminiferous tubule volumes increased significantly in the first 5 months of life and during puberty due to the combined increment in seminiferous tubule diameter and length. The total number of spermatogonia increased until late puberty to stabilize subsequently. Spermatocytes and spermatids appeared during puberty and increased dramatically until the end of this period. The germ cell ratios, indicative of spermatogenic efficiency, improved continuously in late puberty coincidentally with a reduction of spermatocyte degeneration. Sertoli cells proliferated in the neonatal and infantile periods, determining a longitudinal growth of the seminiferous tubules, but remained stable during puberty, when androgen-binding protein secretion increased significantly. The multiplication of germ cells is the main factor responsible for the increment in tubule diameter during puberty and determines the most noticeable postnatal modification of testicular volume. During late puberty, the reduction of spermatocyte degeneration leads to an increment in germ cell ratios and a progressive, but slow, improvement of spermatogenic efficiency, explaining why pubertal development of the testis occurs over such a prolonged period in this primate. This is in contrast to what happens in most laboratory animals and suggests that the Cebus is a useful model for studies of human male puberty.