Using both scanning confocal and electron microscopic morphometric measurements, we analyzed single dendritic spines of CA1 pyramidal cells in the hippocampi of water maze-trained rats vs. controls. Two days after completion of all training, we observed a memory-specific increase in the number of mushroom spines—all of which make synaptic contacts—but not in the numbers of filopodia or stubby or thin spines, as quantified with double-blind protocols in both scanning confocal and electron microscopic images. This memory-specific increase of mushroom spine number was enhanced by the PKC activator and candidate Alzheimer's disease therapeutic bryostatin, blocked by the PKCα-isozyme blocker Ro 31-8220, and accompanied by increases in the number of “perforated” postsynaptic densities, increased numbers of presynaptic vesicles, and the increased occurrence of double-synapse presynaptic boutons associated with the mushroom spines. These and other confocally imaged immunohistochemical results described here involving PKC substrates indicate that individual mushroom spines provide structural storage sites for long-term associative memory and sites for memory-specific synaptogenesis that involve PKC-regulated changes of spine shape, as well as PKC-regulated changes of pre- and postsynaptic ultrastructure.