Experimental hyperoxaluria and calcium oxalate (CaOx) crystals are associated with renal epithelial injury and cell death. A recent study has demonstrated an oxalate-induced increase in cellular apoptosis in vitro, and speculates that this phenomenon may contribute to stone formation. We investigated the incidence of apoptotic cells and the expression of apoptosis related genes in the kidneys of stone-forming rats. Male Wistar rats were administrated ethylene glycol in drinking water and force fed with 1alpha-OH-D3. Apoptosis was detected as a ladder of fragmented DNA in agarose gels of electrophoresed genomic DNA. Apoptotic cells were localized by the terminal deoxynucleotidyl transferase-mediated dUTP-biotin nick end labeling (TUNEL) method. The expression of apoptosis-related genes was analyzed by both reverse transcription polymerase chain reaction (RT-PCR) and immunohistochemistry. While no labeling was detected in the controls or on the first day of administration by the TUNEL method, labeling began to be detected in the renal tubular epithelium of the outer medulla at day 3, and the number of labeled cells increased progressively during the observation period. A ladder of DNA fragments was demonstrated in the kidneys of rats after 2 weeks. Immunohistochemical studies revealed the expression of Fas ligand (Fas L), Bax and interleukin-1 beta converting enzyme (ICE) in the renal tubular epithelium of the descending limb of loop of Henle and the distal convoluted tubules. mRNA of the ICE, c-myc, p53 and Fas L genes was also upregulated in the kidneys of the stone-forming rats.