Dexamethasone exerts a stimulatory effect of rapid-onset on the polymerization of actin. This has been documented in human endometrial adenocarcinoma Ishikawa cells, resulting in an acute, dose-dependent decrease in the G/total-actin ratio. In the present study we completely characterized this fast and apparently nongenomic effect of dexamethasone on actin assembly. We followed the morphological alterations of actin cytoskeleton and measured the time-dependent dynamics of actin polymerization both by ruling out any changes of total actin in the cells and by measuring its transcript. Rapid changes in actin polymerization were accurately measured using a highly sensitive and quantitative rhodamine-phalloidin fluorimetric assay. Ishikawa cells, exposed to 0.1 microM dexamethasone for various time periods up to 24 h, showed a highly significant, rapid, and transient increase in the polymerization of actin starting within 15 min of dexamethasone exposure and lasting 2 h. Treated cells showed a significant (1.79-fold) enhancement of the fluorescent signal compared to untreated cells at 15 min. This value decreased continuously in a time-dependent manner, reaching control levels after 120 min and remained so for the next 24 h. Confocal laser scanning microscopy studies confirmed these findings. Intensive coloration of microfilaments over several scanning sections suggested an enhanced degree of actin polymerization in cells preincubated for 15 min with 0.1 microM dexamethasone. Moreover, actin filaments were more resistant to cytochalasin B. Additionally, quantitative immunoblot analysis showed that the content of total cellular actin remained the same during this period, suggesting that the biosynthesis of actin was unaffected. Northern blot analysis showed that the concentration of the actin transcript was also unaffected. Our data suggest that glucocorticoids induce a fast and self-limited polymerization of actin in human endometrial cells without affecting its synthesis. These findings strengthen the hypothesis that glucocorticoids exert rapid, nongenomic cellular effects and that the actin-based cytoskeleton is an integral part of this pathway, playing an essential role in receiving and mediating steroid signals for the modulation of cellular responses.