The hippocampus is critically involved in spatial learning. Spatial training in adult rats, which improved their spatial learning ability, increased the number of excitatory hippocampal CA1 spine synapses on basal dendrites as compared with either isolated or standardly housed animals (Moser et al. [1994] Proc. Natl. Acad. Sci. USA 91:12673-12675). In this article, we report that spine synapses on oblique apical dendritic branches do not increase in density or number after the same type of training. When examining the variability of the spine density on basal CA1 dendrites by using variance component analysis, the variance associated with the cells was twice as large in all three groups as that coupled to the rats. Analysis of the spine density plots shows that the enhanced spine density after spatial training is found in most cells recorded from the trained group but that a small subset of CA1 neurones are particularly well supplied with spines. The trained group had a significant right-skewed tail of the spine distribution, i.e., training caused high spine density to occur in a small subset of dendritic segments. Conversely, the isolated group had a significant left-skewed spine distribution, indicating that some of the dendritic segments were undersupplied with spines, whereas the paired group displayed no asymmetry.