Shredded wheat moulded by an isolate of Penicillium aurantiogriseum elicited progressive histopathological changes at the rat renal cortico-medullary junction during 5 days of dosing, when incorporated into diet as a 20% component. The changes of acute tubular necrosis and regeneration were seen in the P3 segment of the nephron. In rats exposed to contaminated diet for 5 days the histopathological changes regressed in severity by about one-half within a further 4 days on normal diet and by 7 days the tubular epithelium was nearly normal. A partially purified fraction of an alcohol extract, selected by preparative high-voltage electrophoresis and anion exchange and notably rich in amino-compounds, was typically nephrotoxic when given in diet over 4 days. Acute marked tubular necrosis also occurred when the same fraction was given intraperitoneally over a similar period. The acute histological changes provide a rapid bioassay for this Penicillium nephrotoxicity and facilitate the search for the toxic metabolite(s). The cumulative expression of necrosis and repair over only a few days in tubular epithelium suggests that chronic exposure will elicit a more complex pathology which might serve as an experimental model for the idiopathic Balkan endemic nephropathy.