The effect of testosterone (T) on germinal epithelium development and maturation was studied in prepubertal male rabbits. For this purpose, implants of free T (50 mg) or placebo (PL) were placed intratesticularly in 55-60-day-old animals. Each group was divided into sub-groups additionally treated with 0.9% NaCl, FSH/LH (3.0 IU) or cyproterone acetate (CA, 2.5 mg) over a 45-day period. Initial and final evaluations included measurements of testicular volume, testicular biopsy score count (TBSC) and plasma T, FSH and LH concentrations. At the end of the experiment, increment values (final-initial evaluation) of the parameters examined showed the following differences: (1) mean TBSC in T-implanted, irrespective of additional treatment, was higher than that of PL-implanted rabbits (P less than 0.01); (2) mean plasma T, FSH and LH values were significantly different in T- and PL-implanted rabbits, with higher T (P less than 0.05) but lower FSH and LH increments (P less than 0.01 for both) in the former group; (3) marked differences amongst types of additional treatment, irrespective of the implant used, were found for TBSC, T, FSH and LH (P less than 0.01 for all); animals treated with CA had markedly lower increments than other treatment groups. From these findings, it may be concluded, that in the prepubertal rabbit T plays an important role in development and maturation of the testis, but this effect probably requires the synergistic action of endogenous gonadotrophins.