Biologists often need to know how different actions affect the health of a culture of cells. One important approach is to measure the rate at which cells are converting sugars into energy via aerobic respiration. MML researchers have developed a scanning electrochemical microscope (SECM) to measure the respiration rate of single cells within sub-confluent cultures. SECM uses a microelectrode (Pt) that is scanned about 10 um above adherent cells. Stationary reference (Ag/AgCl) and counter (Pt) electrodes are also inserted into the cell culture media. The scanning electrode's potential is set for the particular species to be measured and the resulting current is measured as a function of position. The entire SECM apparatus is mounted on the stage of an inverted optical microscope so that images of the scanning electrode's position and the cells can be made. The respiration rate of each cell in an optical image can be quantified from the corresponding SECM image.