A new and highly sensitive method is described forinvivoandinsitustudies of photosynthetic activity of undetached leaves based on the photoacoustic effect. The general utility of this simple photothermal method is illustrated by examining the spectroscopy and photosynthetic activity of green, striped, and albino plants of maize. The technique is also used to show that the oxygen evolution in hybrids is higher than in inbred lines of maize.