Although the collared peccary (Tayassu tajacu) is found throughout the Americas, with a high potential for domestication and commercial exploitation, there are few data on the reproductive biology of this mammalian species. The aim of the present study was to investigate testis structure, spermatogenic cycle length, Sertoli cell efficiency, and spermatogenic efficiency. Twelve adult peccaries were used for biometrical, histological, and stereological analyses; 3 of these peccaries received intratesticular injections of (3)H-thymidine for the determination of the duration of spermatogenesis. Testis weight and gonadosomatic index were 23.7 +/- 1.8 g and 0.2% +/- 0.1%, respectively. Seminiferous tubule volume density was 77.4% +/- 1.7%. Leydig cells occupied 12.8% +/- 1.8% of the testis parenchyma and presented a peculiar cytoarchitecture in the periphery of the seminiferous tubule lobes. The premeiotic, meiotic, and postmeiotic stage frequencies were very similar to those found for wild and domestic boars. The spermatogenic cycle and entire spermatogenic process (based on 4.5 cycles) lasted approximately 12.3 +/- 0.2 and 55.1 +/- 0.7 days, respectively. Daily sperm production per gram of testis in the collared peccary was approximately 23.4 +/- 2 x 10(6), which is similar to that of domestic and wild boars. The knowledge generated in the present study could be used in reproduction and animal improvement programs and provides important information that may be used for comparative reproductive biology with previously investigated mammalian species.