Experimental ischemic acute renal failure results in disruption of proximal tubule apical membranes. Previous work utilizing immunofluorescence with an anti-actin antibody has demonstrated that the apical cytoskeleton of proximal tubule cells is disrupted during ischemic injury. In this study, using rhodamine-phalloidin which stains only filamentous actin, we demonstrate that graded durations of ischemia resulted in progressive disruption of proximal tubule apical microfilaments. Quantification using spectrofluorometry showed that 5, 15 and 50 minutes of ischemia resulted in 32.8 +/- 4%, 48.8 +/- 2.5%, and 58.4 +/- 2.6% decreases in apical F-actin relative to controls. Ischemia did not qualitatively affect either glomerular or distal tubule F-actin structure, though there were nonprogressive increases in glomerular fluorescence. In summary, rhodamine-phalloidin staining can be used to qualitatively and quantitatively assess proximal tubule microfilaments in vivo. We conclude that ischemia results in very early loss of proximal tubule apical microfilaments, with the majority of F-actin loss occurring within five minutes.