When CuCl2 is allowed to act on Aspergillus niger there is at first a period during which there is no change in the rate of the production of CO2, following which the rate of respiration falls. The interval of no change is called the latent period. When the Cu is removed from the suspension medium before the end of the latent period, this interval is prolonged. The rate of CO2 production then falls to a new level, a definite distance below the normal level, indicating that the Cu has been active in the cell during the time of exposure although no effect was observed until after its removal. The length of the latent period varies inversely as a constant fractional power of the Cu concentration. Using other criteria of toxicity, it was determined in the case of Nitella and Valonia that Cu penetrates the cell almost immediately and it may be assumed that it penetrates likewise with Aspergillus. The results are explained by assuming that the Cu is activated in the respiration system by means of a reversible reaction. By using appropriate velocity constants the experimental curves can be duplicated by calculated curves.