Changes in small intestinal morphology (jejunum) were examined at 28 d of age in chicks fed with full-fat sunflower kernels (FFSK)-based diets. Jejunal mucosa of chicks (six chicks per treatment) were embedded in Epon-812 for examination by a conventional electron microscope procedure. A portion of the tissues was also sectioned and embedded in paraffin for examination by light microscopy. Brush border, goblet cells, and intraepithelial lymphocytes, on the one hand, and fibroblast and mononuclear cells (lymphocytes, plasma cells, macrophages) were observed in the enterocytes and lamina propria, respectively. The results showed that the birds fed 150 g kg(-1) of FFSK showed a shortening and thickening of the villi, hyperplasia and vacuolar degeneration of enterocytes, and hypertrophy and hyperplasia of goblet cells. Likewise, an increment of intraepithelial lymphoid cells and hypercellularity of the lamina propria was observed. In addition, electron microscopy showed large vacuoles in the enterocytes, which could be dilations of agranular and granular endoplasmic reticulum or Golgi. There were many dark granules within the vacuoles that could be triglyceride-rich lipoproteins (portomicrons). These lesions could have been due to the presence of chlorogenic acid or to the greater concentration of oil in the FFSK diet. Our observations demonstrated that addition of 150 g kg(-1) FFSK to broiler chicken diets caused alterations in jejunal mucosa that could explain the decrease in fat digestibility observed in a previous experiment in which we incorporated FFSK into broiler diets.