Objectives: To determine if thimerosal administration in amounts equivalent to vaccines content produces neurotoxic effects on the encephalon in postnatal hamsters and on experimentation animals' development. Design: Experimental, prospective, bietapic study. Setting: San Fernando Faculty of Medicine, Universidad Nacional Mayor de San Marcos. Biologic material: Seven-day old hamsters. Material: We divided 45 postnatal hamsters in three groups: group A (n = 15), group B (n = 15) and group C (n = 15). We administered three intramuscular equivalent doses of sucrose and thimerosal in 20 μL of physiological serum respectively to groups B and C on birth-days 7 (0,227 μg), 9 (0,216 μg) and 11 (0,220 μg). Group A received only 20 μL of saline solution. Main outcome measures: Body weight, encephalon weight, hamster's stature and encephalon histopathological alterations. Results: Anova and st...