We investigated the effect of brefeldin A(BFA)on the Golgi apparatus in cultured tobacco protoplasts. Within 3 hr of treatment with brefeldin A(10μg/ml), the diameter of the Golgi cisternae became about four times greater than the control. The expanded cisternae were loosely organized, their tips were sprinkled with ribosomes and the distance between the cisternae increased. No other significant morphological changes were detected, except the appearance of many vacuoles. From these observations it was concluded that BFA causes morphological changes in Golgi apparatus.