Air-dried mycelium of Penicillium aurantiogriseum2, grown as a surface culture on yeast extract-sucrose medium, was incorporated into powdered diet and fed to rats and hamsters for different periods up to 28 days. At intervals, animals were anaesthetized and the kidneys fixed in situ by perfusion. In rats, the fungus produced scattered exfoliation of pyknotic cells and an increased frequency of mitotic figures involving the pars recta segment of proximal tubular epithelium. This lesion was detectable as early as three days after beginning of treatment and was well developed by 14 days. No degenerative tubular change or mitogenic effect was observed in hamsters, even after feeding for 35 days; and there was no apparent renal pelvic or interstitial lesion in either species.