The biochemical and ultrastructural aspects of intestinall fat absorption and chylomicron formation have been studied in rats treated with actidione-cycloheximide. Rats were prepared with a catheter in the main mesenteric lymph duct. The next day, primor rapeseed oil containing 14C triolein was infused in the duodenum of control rats and actidione-cycloheximide treated rats four hours before experiments. In each case, the quantity of lipids absorbed was determined after six hours, and the lipids present in the mucosa were studied as were those present in the lymph collected throughout the absorption period. Furthermore, the ultrastructural study of the mucosa and the morphological study of the lymph lipoprotein particles were carried out using electron microscopy. Disappearance of intraluminal lipids and mucosal lipids appeared slightly impaired after actidione-cycloheximide treatment, but only a very small proportion of the infused lipids were recovered in the intestinal lymph (0.05% compared to 3% in control rats, Table I). The amount of esterified lipids found in the mucosa, after six hours, was clearly lower (49%) when compared with those of control rats (79%) (Table II). This suggests that, following inhibition of protein synthesis by actidione-cycloheximide, lymph fat absorption was much impaired. An alteration in cellular structure appeared in microscopic observations of treated rat enterocytes (Fig. 2 and 3) compared with the control (Fig. 1). Many whorl formations and autophagic vacuoles were included in these cytotoxic effects during lipid absorption. Treated animal enterocyte lipid particles did not have the organized structure of those in the controls. While lymph particles of control rats revealed a regular progression in size (Fig. 4A, B), lymph particles of treated-rats showed a few large chylomicrons (equal or far larger than in the controls) among small particles (Fig. 4 C, D, E); furthermore, evidence of coalescence might be observed (Fig. 4 F). All these observations emphasize the prevailing role of the different protein structures of the enterocytes during lipid intestinal absorption, particularly for chylomicron organization after long-chain fatty acid uptake.