Estrogen (E) treatment of ovariectomized animals increases dendritic spines and/or synaptic protein expression in the hippocampus of female rats [J Neurosci 12 (1992) 2549; Endocrinology 142 (2001) 1284; Endocrinol Rev 20 (1999) 279; Annu Rev Pharmacol Toxicol 41 (2001) 569], mice [Proc Natl Acad Sci USA 101 (2004) 2185], rhesus monkeys [Proc Natl Acad Sci USA 98 (2001) 8071; Endocrinology 144 (2003) 4734; J Comp Neurol 465 (2003) 540] and hippocampal cells in vitro [J Neurosci 16 (1996) 4059; Neuroscience 124 (2004) 549]. The role of E in hippocampal synaptic structural plasticity in males is less well understood. In the present study, we have used a recently developed technique to count spinophilin immunogold-reactive (Ir) puncta as well as in situ hybridization to compare E effects on spinophilin-Ir and mRNA in gonadectomized female and male rats 48 h after E treatment. Spinophilin is an established spine marker, which interacts with several proteins (including actin and protein phosphatase 1) that are highly enriched in spines [Proc Natl Acad Sci USA 94 (1997) 9956; Proc Natl Acad Sci USA 97 (2000) 9287]. We report that E exerts sex-specific effects on dendritic spinophilin-labeled spines in the CA1 region: E treatment significantly increased spinophilin-Ir puncta, indicative of spines, in females, but led to a decrease in males. Furthermore, while hippocampal spinophilin mRNA changes could have occurred earlier, spinophilin mRNA levels were unchanged after 48 h of E in both males and females. This suggests the possibility that E regulates spinophilin protein expression and or stability within dendrites via post-transcriptional mechanisms.