The typical renal histopathological changes in proximal tubules of rats consuming food containing shredded wheat moulded by Penicillium aurantiogriseum did not occur in neonates of rats ingesting the nephrotoxic diet during pregnancy nor in pups fed only by lactation from a treated dam. In contrast, weanlings consuming the moulded diet consistently showed within a few days densely staining mitosis-like structures in the proximal tubules which was the first step leading to development of the prominent tubular cytomegaly and karyomegaly seen when 16 weeks old. Karyomegaly and cytomegaly became evident also when mouldy shredded wheat constituted only 1% of a diet consumed on 45 days during the first 8 weeks after weaning, demonstrating the potency of the active component. Relevance to the putative involvement of nephrotoxic mycotoxins in human renal disease is discussed, as is the apparent absence of a renal histopathological response in adult rats during pregnancy and lactation.