Changes in spine morphology may underlie memory formation, but the molecular mechanisms that subserve such alterations are poorly understood. Here we show that fear conditioning in rats leads to the movement of profilin, an actin polymerization–regulatory protein, into dendritic spines in the lateral amygdala and that these spines undergo enlargements in their postsynaptic densities (PSDs). A greater proportion of profilin-containing spines with enlarged PSDs could contribute to the enhancement of associatively induced synaptic responses in the lateral amygdala following fear learning.