Gelsolin is a Ca2+ and polyphosphoinositol-phospholipid regulated modulator of actin polymerization present in most mammalian cells and in plasma. Cytoplasmic gelsolin was first described in highly motile cells such as leukocytes, where actin polymerization is dynamic; however arterial smooth muscle cells (SMC), despite their stabilized actin bundles, express high levels of gelsolin. We have investigated gelsolin modulation in rat aortic SMC by immunohistochemistry, Western blotting and Northern hybridization using three models which are known to show modulation of actin isoform expression: development, aortic intimal thickening after experimental endothelial injury, and growth in culture. When related to the protein and mRNA content of adult aortic SMC, gelsolin is expressed about 50% in aortic SMC of five-day-old rats, 20-30% in SMC of intimal thickening 15 days after endothelial injury (when SMC are actively replicating) and in SMC growing in culture; in intimal thickening 60 days after injury (when SMC have returned to quiescence), the gelsolin content becomes similar to that of control SMC. The high level of gelsolin in smooth muscle (SM) tissues and the down regulation with proliferation and migration raises the question as to whether gelsolin in these cells has functions other than the dynamic control of actin filament length. The similar modulation patterns of gelsolin and alpha-SM actin suggest a preferential interaction between these two proteins.