Individual and age-dependent variations in the venoms of 23 specimens of Bothrops atrox were studied using SDS-polyacrylamide gel electrophoresis, isoelectric focusing and different enzymic activity measurements. The electrophoretic patterns showed a decrease in total protein band numbers and a shifting from components with higher to those with lower molecular weights with increasing age of the snakes. The venoms of individuals younger than 12 months showed isoelectric focusing patterns within a pH range of 3.5-6.0, whereas those of older specimens extended from pH 3.5 to 7.0. Individual variations were found in fibrinogen-clotting and fibrinolytic activities. Despite the fact that important quantitative differences were also found in amidolytic activities measured on five chromogenic proteinase substrates, all the venoms showed the same activity profile. Specimens of the oldest age group (8 years) showed absence of some enzymatic activities.