Previous research on semifree-ranging mandrills has shown that the degree of secondary sexual development differs among adult males. While some males are social, brightly colored, and have large testes and high levels of plasma testosterone, other males are peripheral or solitary, and lack fully developed secondary sexual features. In order to determine how these differences among males arise, and to investigate the influence of social factors, we examined the adolescent development of 13 semifree-ranging male mandrills of known age. Testicular volume began to increase markedly at 5.5 yr, and males began to develop secondary sexual adornments at the age of 6 yr. Males attained adult size and secondary sexual development at an average age of 9 yr. As males developed, they peripheralized, decreasing from 100% group-associated at 5 yr to 20% at 8 yr. At 9 yr some males reentered the social group and attained alpha rank, while others remained peripheral or solitary. Within this average development, there was marked variation among males in the timing of development. Adolescent males that were dominant for their age had higher testosterone levels, larger testes, and more advanced secondary sexual development than subordinate males. The implications of these findings are discussed in the light of differences that occur among adult males, male–male competition, and the evolution of secondary sexual adornments in this species. Am. J. Primatol. 56:9–25, 2002. © 2002 Wiley-Liss, Inc.