Continuous or intermittent consumption by rats of food moulded by Penicillium aurantiogriseum induced prominent and extensive histopathological changes within several weeks seen specifically at the renal cortico-medullary junction. Many cells of the P3 segment of proximal tubules contained either giant nuclei or multiple enlarged nuclei, described in this text as karyomegaly, but also included within a cytomegalic change. The changes contrasted with the tubular cell necrosis and concomitant mitosis elicited after only four days consumption of nephrotoxic mould. Unilateral nephrectomy enabled persistence of histopathological changes to be assessed directly after detailed histology at an earlier stage. After ten days consumption of food with a 100-fold excess of fungal extract containing the amphoteric nephrotoxins, the typical acute histopathology evolved, over a period of three weeks on normal diet, into the bizarre karyomegalic histopathology, implying a latent effect. Karyomegaly persisted for at least twelve months after nephrotoxin dosage ceased. P. aurantiogriseum karyomegaly was much more striking than that induced by a relatively high chronic dose of another Penicillium nephrotoxin, ochratoxin A. Although the study does not attempt to measure relative potencies, qualitatively similar ultrastructural changes (enlarged nuclei, proliferation of smooth endoplasmic reticulum and thickening of proximal tubule basement membranes) were induced by the two types of nephrotoxin. The broadly toxic ochratoxin A is the popular putative aetiological agent in the mysterious and insidious Balkan endemic nephropathy and associated urinary tract tumours. As the renal carcinogenicity of ochratoxin A in rats follows karyomegaly, the striking karyomegaly induced by P. aurantiogriseum in the proximal tubules of the kidney must be considered as a potential factor in human chronic renal disease.