The effect of Alanap, N-1-naphthylphthalamic acid (NPA), on the permeability of onion {Alliuni cepa) epidermal cells was studied by means of a plasmometric method. Urea, 1,3-dimethylurea, and water were used for the test of permeability.It was found that the treatment of onion epidermis with NPA at various concentration (5x10--^ to 5 x 10-*^ M) altered the deplasmolysis rates and the absolute permeability constants. However, NPA did not alter the osmotic value of the treated cells. It was suggested that NPA treatment altered the permeability of onion epidermal cells due to the effect of this herbicide on the plasma membrane. K. A. FENG Physiol.Plant. 32.1974