PURPOSE. To examine the histologic and ultrastructural changes in Bruch's membrane (BM) in apolipoprotein E deficient [ApoE(-)] mice in comparison with age-matched control animals. METHODS. Two-month-old (group 1) and 8-month-old (group 2) normal control C57BL/6 mice and 2-month-old (group 3) and 8-month-old (group 4) ApoE(-) mice were studied. All groups of mice were fed a standard rodent diet. The mice were killed, serum lipid levels were determined, and the eyes were ultrastructurally examined using standard techniques to measure the thickness of BM. The area fraction of electron- lucent (EL) particles in BM was quantified using point-counting stereology. RESULTS. The serum cholesterol levels of the ApoE(-) mice were significantly higher than those of the control mice (P = 0.0001). There was a significant thickening and EL particle accumulation in BM associated with age in the control animals. Group 2 had a thicker BM and more EL particle accumulation than group 1 (P = 0.0410 for thickness; P = 0.0042 for particle accumulation). Age-related changes were not seen in ApoE(-) mice; thickness and accumulation were similar in groups 3 and 4 (P = 0.50, thickness; P ≃ 1.0, accumulation). Significant thickening and accumulation were seen in young ApoE(-) mice (group 3) versus young control animals (group 1; P = 0.008, thickening; P < 0.0001, EL particle accumulation). Group 4 ApoE(-) mice did not have a thicker BM or more EL particles than group 2 control animals (P = 0.2910, thickness; P = 0.35, EL particle accumulation). 'Membrane-bounded' material (material between two membranes) was present significantly more frequently in ApoE(-) mice. CONCLUSIONS. ApoE(-) mice exhibit accumulation of EL particles at an earlier age and have more membrane-bounded material in BM than control mice. This material has ultrastructural similarities to basal linear deposit, which accumulates in age-related maculopathy.