The purpose of the present study was to conduct a light and electron microscopic investigation of pathological alterations of the kidney during the recovery phase after injury caused by mercuric chloride toxicity, and the morphological changes in regenerating cells influenced by subsequent mercuric chloride toxicity. The first administration of mercuric chloride to female albino rabbits was performed by intravenous injection at the dose of 1.3 mg/kg, and a second similar injection was given 14 days later. Regenerating cells first appeared 3 days after the first administration and gradually increased thereafter. However, maturation of these cells was incomplete even on the 14th day. After re-administration, the degree of tubular necrosis was significantly reduced compared with that after the initial injection, and most parts of the basement membranes of the degenerated tubules were covered with epithelial cells. Electron microscopy showed that almost all of the cells underwent changes, including disrupted microvilli, irregularly dilated rough endoplasmic reticulum, and dense free polyribosomes arranged as rosettes. Moreover, S-phase cells were found adjacent to the cells. These findings suggested that the regenerating cells were influenced again by the toxic effect of mercuric chloride, but escaped cell necrosis and remained within the reversible stage because of their immaturity and reduced sensitivity to mercuric chloride.