Dendritic spines are small protrusions from dendrites on which the majority of excitatory synapses occur (Gray, 1959; Peters et al., 1976; Harris and Kater, 1994). Dendritic spines serve to compartmentalize second messenger systems (Koch and Zador, 1993) and are fundamental to synaptic transmission, long-term potentiation, and long-term depression (reviewed by Matus, 2000). As such, spines are thought to be the locus of memory formation (Moser et al., 1994; reviewed by Segal et al., 2005), and their pathology is seen in diseases characterized by a decrease in mental capacity such as Alzheimerís Disease (Ferrer and Gullotta, 1990) and Fragile X Syndrome (Hinton et al., 1991; Wisniewski et al., 1991; Irwin et al., 2000; Nimchinsky et al., 2001). Thus, dendritic spines are critically important to neuronal function (reviews by Harris and Kater, 1994 and Shepherd, 1996). Spines have a highly dynamic actin cytoskeleton. In the past, this cytoskeleton has been regarded as merely a support structure. However, recent work from this and other laboratories identifies a much more functional, even regulatory role for actin within the spine. In the current series of experiments, we demonstrate the following aspects of spine formation and actinís role in said process: (1) spines proliferate in acute adult hippocampal slices (2) protein synthesis is essential for slice spine proliferation, (3) actin is upregulated during spine formation in the juvenile acute slice, and is thus regulated as opposed to constituitively expressed, (4) actin overexpression leads to an increased spine density in the organotypic slice culture, largely the result of an increased number of elongated spines (5) all spines have a large degree of actin turnover in less than twenty-four hours.