Pantaneiro horses are a breed native to flood plains of Brazil, where they thrive with little human interference. The aims of this study were to characterize age-associated changes in testicular size, serum testosterone and sexual behavior and to determine age at puberty as well as to evaluate daily sperm production in the Pantaneiro stallion. After weaning the males were kept in bachelor groups away from the females, in a separated area in the natural environment of flood plains. Infantile period ended at approximately 14.4 m.o. of age. Sexual interest and breeding capability first appeared between the ages of 15.6 to 27.5 m.o. The transition from prepuberal to postpuberal histological stage, occurred at approximately 27.8 m.o. of age. All animals were in the postpuberal period at 38.5 m.o. of age. Daily sperm production estimated with testicle homogenate, at 38.5 m.o. of age was low (1.4603 +/- 0.48 x 10(9) sperm) and serum testosterone levels were similar to adult levels by 27.8 m.o. In conclusion the 2 y old Pantaneiro stallions showed full sexual behavior and had adult testosterone levels, but at this age, the stage of development of the seminiferous epithelium was variable and the efficiency of sperm production was still low.