Several observers have reported that leaf transpiration is increased with many plants by the presence of copper sprays. It has also been quite definitely proven, especially by Wilson and Runnels (4) with Coleus, that the greatest increase takes place at night. Little data, however, have been gathered to explain this phenomenon. This paper reports certain experiments carried out with tomato plants which both substantiate the findings of earlier workers as to the increase of transpiration rates due to copper sprays and give some evidence as to the mechanism by which it takes place.