Patulin is a mycotoxin that is produced by species of Penicillum, Aspergillus, and Byssochylamys molds that may grow on a variety of foods including fruit, grains and cheese. Patulin, at a dose of 0.1 mg kg(-1) bw day(-1) was administered orally to growing male rats aged 5-6 weeks for a period of 60 or 90 days. The dose of patulin used in the present study was based on estimated human exposure levels. At the end of these periods, the thymus glands of patulin-treated and control Wistar rats were removed and ultrastructural changes in capillary cells of the thymus of patulin-treated Wistar rats were determined by electron microscopy. The walls of thymus capillaries of the 60-day patulin-treated rat groups (P-60) exhibited degeneration observable in electron microscopic sections. For example, loss of cytoplasm and mitochondrial cristae of cells, swollen endothelial cells, increased thickness of the basement membrane, closed lumen of capillaries, accumulation of fibrous material at the periphery of the capillaries and nuclear anomalies were seen in these sections. Such degeneration and changes were also observed in sections of capillaries of the 90-day patulin-treated rat groups (P-90). The levels of degeneration of endothelial cell nucleus of P-90 were greater than those of P-60. This study demonstrated the ultrastructural degeneration of thymus capillary cells of patulin-treated rats. The results obtained from this study may provide a guide to research dealing with the toxic effects of patulin on tissue and organ ultrastructure. Copyright (c) 2005 John Wiley & Sons, Ltd.