Apolipoprotein E4 (apoE4), one of the three most common human apoE (h-apoE) isoforms, is a major genetic risk factor for Alzheimer's disease and for cognitive deficits associated with aging. The biological mechanisms involving apoE in learning and memory processes are unclear. A potential isoform-dependent effect of h-apoE on cognitive performance was studied in gene-targeted mice, which show physiological expression levels and distribution of h-apoE3 or h-apoE4. Male and female h-apoE3 and h-apoE4, apoE-deficient and C57BL/6J mice (4-5 months) were subjected to tasks evaluating spatial memory and avoidance conditioning. Female h-apoE4 mice did not detect changes in the spatial configuration of objects as opposed to female h-apoE3 mice. Female h-apoE3 mice failed to improve their performance during training in a reference memory version of the spatial water-maze task, but performed well during the probe trial 24 h after the last training trial. Memory retention performances of h-apoE4 mice were impaired during this probe trial. Both h-apoE3 and h-apoE4 mice did not improve their performance in a water-maze delayed matching to place task. Finally, h-apoE3 mice showed mild perturbations in a Y-maze active avoidance task, whereas both h-apoE mouse lines performed well in a passive avoidance task. Thus, spatial memory performances appeared particularly sensitive to h-apoE-isoform-dependent effects. Deficits occurred predominantly in female h-apoE4 mice, which support the hypothesis that humans carrying h-apoE4, especially women, have impaired spatial memory compared to those carrying h-apoE3.