Effects of brefeldin A (BFA) and nordihydroguaiaretic acid (NDGA) on endomembrane structures and lipid synthesis were compared in maize root cells and tobacco Bright Yellow-2 cells. Immunofluorescence and electron microscopy studies showed that NDGA altered the structure and distribution of the endoplasmic reticulum (ER) within 1 h but not of the Golgi apparatus whereas, as shown previously, BFA altered that organization of the Golgi apparatus and, only subsequently, of the ER. Biochemical studies revealed that both drugs and especially BFA led to a strong inhibition of the phytosterol biosynthetic pathway: BFA led to accumulation of sterol precursors. The importance of phytosterols in membrane architecture and membrane trafficking is discussed.