The sequence of morphological changes during foot process effacement in acute puromycin aminonucleoside (PAN) nephrosis was examined by means of NaOH maceration and freeze cracking for scanning electron microscopy (SEM). The micrographs of SEM and those of transmission electron microscopy (TEM) were quantitatively analyzed by computerized morphometry, and were correlated with renal function. On day 2 after PAN injection, the slit length was moderately decreased by both shortening and degradation of the foot processes. On day 4, membrane-bounded vesicles were scattered in the lamina rara externa. During foot process effacement, the basal surface of podocytes developed palm-like domains that represented the cytoplasmic areas between interdigitation. The decrease in the length of podocyte cell borders paralleled the decrease of 24-hour creatinine clearance. The development of the palm-like domains on the basal aspects of podocytes estimated by distance class analysis was closely correlated with the sudden onset of proteinuria. We conclude that foot process effacement in PAN nephrosis caused by the retraction and degradation of foot processes leads to the development of palm-like domains, which is correlated with podocyte detachment as well as massive proteinuria.