In “Three-Dimensional Structure of Dendritic Spines and Synapses in Rat Hippocampus (CAl) at Postnatal Day 15 and Adult Ages: Implications for the Maturation of Synaptic Physiology and Long-term Potentiation” (K.M. Harris, F.E. Jensen, and B. Tsao; J Neurosci 12:2685–2705, July 1992), figures 9C and 9E were inadvertently switched with one another. Figure 9C is the electron micrograph of the reference section of the spine reconstructed in figure 10E, and figure 9E is the electron micrograph of the reference section of the spine reconstructed in figure 10C. The authors regret this error.